Welcome to the forum, @centaur, and thanks for posing the question.

Although it's an interesting topic to cover, I think we're in danger of going off down a potentially misleading rabbit hole. The question you've asked is basically assuming you'll need a pretty hefty heat pump and as a rule you should only plan for a large unit if you've already confirmed it's necessary. I know you're talking about a house that predates cavity wall insulation but that still doesn't necessarily mean the heat pump will always be chewing through oodles of leccy - our own @cathoderay has been successfully running a heat pump for a number of years now in just such a property.

Has anyone actually done a heat loss survey yet on your home? Alternatively, have you used the HeatPunk online application and done a heat loss survey yourself? Neither, of course, is a guarantee of how your home will actually work but it should give you a close enough range to be able to sort out the potential installers who're talking sense from those who're stringing you along. It will incidentally also give you a much better insight into what about your home really matters and what's a bit of a red herring.

Bear in mind that the boiler you're talking about may never have needed to put out its peak power and even then likely only to warm up rooms that had been allowed to cool significantly before their next blast of hot air. My old oil boiler was exactly that model and has now been replaced with a 8.5kW Mitsubishi Ecodan (and, in fact, a 7kW Vaillant was specified originally but unfortunately suffered from supply chain issues). In other words, it's all about how much power is needed to maintain a comfortable temperature rather than how much power it can throw at your home in one go.

Are you able to give any more detail about the kind of property, the number of bedrooms, the square footage and/or your annual oil consumption? That'd be really useful for letting us all get a feel for the kind of heating requirements your installers would be looking at.

Further to that which @majordennisbloodnok explained, there is also the likelihood that the old system was run at high heat settings but not for a 24/7 regime. Because the old idea was to blast   out very high levels of heat for short periods of time - rather than a steady much lower level of heat, the 20+ kW rating may be well over the requirement for a properly run heat pump regime. Regards, Toodles.

As per the above, first you need to find out what the demand (heat loss) actually is.

If you can tell us your annual fuel consumption, roughly your heating regime and floor area it's possible to do a ballpark good enough to work out if you may need to consider a cascade.  Then you will need a proper survey which, as stated above, you can do yourself of have done.

As above, you need to know the actual heat loss. The fact a xxkW oil boiler was fitted tells you very little, in the past it was not uncommon to 'solve' the heat loss calculation conundrum by fitting boiler with so large an output it couldn't fail to supply enough heat. Control was by thermostat controlled on-off running (plus or minus a timer) so the over sizing didn't matter so much, the boiler just spent more time off if it's output was too high.

Heat pumps on the other hand almost always use something called weather compensation, which adjust the flow (heat pump output) temperature according to the outside air temperature (OAT). The lower the OAT, the higher the flow temp, the idea being the compensation works in such a way that the heat put into the house matches the heat lost by the house throughout a range of OATs. It generally does this with the heat pump on all the time, and furthermore, there is no point at all (not to mention some minus points) in having an over size heat pump. Not only will it cost more, you will also  find it can go down low enough when it is warm outside and the flow temp needs to be much lower.

I went from an oil boiler (only a bit over size - I did the design (including heat loss calcs) and installation myself so no need to 'solve' the heat loss conundrum by fitting an over size boiler) to a heat pump, all in an old leaky Grade II listed building (has Listed Building Consent and so Planning permission implications), albeit not that large, and it has worked out fine. My calculated heat loss by my heat pump installer was just over 12kW, a bit more than the heat loss I worked out for the oil boiler, and the actual measured heat loss is under 10kW. My heat pump is a Midea 14kW R32 unit, but that is the badge output, on cold days it drops to just over 11kW, only just a bit over what I need. Midea also do (o did) a 16kW version. Its actually the same hardware, just with the brakes taken off in the software by flicking some dip switches. I think they now do a 16kW R290 version.

Some people who should know better will tell you you can't use a heat pump to heat buildings such as the ones we have. It's nonsense, a building doesn't care where the heat comes from, all it needs is enough heat. I have that, and all is well. That said, my heating bills will always be higher than a modern well insulated building, whatever the heat source is, because I have an old leaky building, but that's OK because the old sandstone walls are very attractive!

A small but important technical point: if you use one of the heat loss calculators, they tend to over-estimate heat loss through old stone walls. We can go into further detail if needs be in due course.    

Determining heat loss using past oil consumption is very approximate, because we rarely have accurate tanks levels at key points or set intervals, and deliveries can be very varied in both timing and amount. If you can guesstimate your annual oil consumption, then, as @jamespa says, given your pattern of heating use (overall heating season, whether timed or continuous over 24 hours), then we can guesstimate your heat loss.

Lastly, be warned, you may find you use more heat when switching to a heat pump. I did and still do. It happens if you switch from timed heating (which a fossil fuel can manage, but a heat pump can't) to always on heating (which is pretty much the only way to run a heat pump). The far far greater heat pump efficiency helps reduce the impact, and you will feel more comfortable, and dampness and condensation will become things of the past.  

Uh, cathodeRay was faster .... anyway:

I rarely saw any oil burner below 20 kW independent of the building's real heat loss .
This means, nearly all oil boilers in family homes are between oversized and grossly oversized.
Afaik this has to do, from a design point of view, with the burner's spray nozzle which neither can't made smaller without a danger of clogging nor being run with lower oil pressure since this would impair its spray pattern. There were ambitious concepts in the past but never became common.

While burning fuel, oversizing is no serious matter, but heat pumps act differently and oversizing them will cause either trouble, serious trouble or endless trouble. So, there's urgent need to size correctly.

As a first throw, take the oil consumption of an average year in litres and divide that figure by 300.
The result in kW will be a not too rough guessing and provides you with a feeling of what is really needed.
As an example, my parental home runs with 3000 l per anno, so a 10 kW heat pump would fit at first sight.
With all the original single pane windows it was more like 4000 l, so 13 kW.
When in future the remaining single panes will get replaced (especially the 60's shop window pane in the living room which will need to be parted with a jamb halfways-- my mother says 'Don't do that as long as I live') it will more like 2000 l, so 7 kW. With a new roof and some wall insulation it would be even less.
That approach is called 'Weiersmüller' and gets used in Switzerland with good grace since roughly 1980 when oversizing was no concern.
Also Swiss winters are different from English ones, so operating with a denominator of 400 may be an option. In my parental home this would come down to 10, 7 or even 5 kW.

Especially when your house features that kind of pencil-sized pipework which gladly was rolled out since the 90's, it will be safer to keep to an even smaller heat pump which will not fully cover the, let's say, ten coldest day of a year, because otherwise the thing may have trouble otherwise to distribute the energy into the system. This would mean you need an oil filled radiator or like to add some little extra heat in icy January nights, but better this than having a heat pump with persisting hiccough or replacement of half of the installation.
I myself love a fan blower in the bathroom to push temperature up to 25°C while being under the shower (me, not the blower) and later drying my feet in the hot wind while toweling my sparse hair; so in our household such a tool is already in place.

When layout is done by your installer, he will avoid at any rate that you get cold during Twelfthtide and try calling him from his well-earned vacation, so he will reach to size bigger, unaware of other problems. To be safe, he may rely on the heat pump supplier's support, where an admin puts information easy to obtain (house size, build, year of construction), provided by the installer, into a computer program and comes back with a result 'on the safe side'. The installer himself may add 'further safety' and if you insist on keeping warm at any time might top up once again. The forums are full with hair-raising stories of oversized heat pumps which can brought to collaboration only with guile and wile by an interested community while the initiators have withdrawn into their rabbit holes and are difficult to hold responsible.

Even when you get advice here, it will be a problem to find an installer who is ready to follow your ideas; indeed he's the expert and will be accountable for any result unless you sign him that he's not (and even then a judge may find that he didn't advise you well and hold him liable).