@Gordonville, it sounds like you're going to end up with a completely new system, which it what we had.  We started with no central heating at all.  UFH works well with an ASHP but you don't need it. It is a different sort of heating and is also quite expensive to install so don't discount radiators. We used all standard 15mm copper pipes in our system.

Our fuse was OK as it was, because our supply was previously running 8 storage heaters. You do need to ask your Distribution Network Operator (DNO) before you install a ASHP; they will tell you whether the incoming supply is up to it (it probably will be).  Upgrading the fuse is then not a big job if needed.   

That's quite a high heat demand for a semi but hopefully your insulation work will reduce it. You will end up paying more than with gas but I'm sure you know that.

You can download the MCS heat loss calculator here.

https://mcscertified.com/mcs-launch-new-improved-heat-pump-calculator/

Do check the permitted development rules for ASHPs as the Scottish ones are a little more restrictive on where you can site the ASHP.

Good luck and keep us up to date with progress.

@gordonville, the one thing that I can comment on, and that you've already highlighted, is the bore and rad size. It took us a while to get the bottom of why the southern end of the house wasn't coming to temperature and we finally ascertained that the reduced pipework work from the utility room where our tank is to the southern portion of the house was causing the issues, so the upgrade to K3 rads from Stelrad was a game changer. The true test is still coming, but I'm confident we'll be far warmer this winter.

Have you received any quotes yet, and do you have a brand preference at this stage?

Thank you for the replies. That MCS heat loss calculator looks like a really powerful tool. I’m kind of concerned that neither of the quotes we have had go in to such detail. 

We had one quote for a Vaillent 12kW high temp unit and tank with UFH.
Another quote was for a Nibe F2040 12kW with Telford tank and UFH. 

Both came in at just over 20K. Seems like too much. We are learning as we go and this forum really helps with that. 

I used the EST search bar to fire out loads of further quote requests and have had a couple of replies. It has helped me get the appropriate information together. Lots of companies seem very busy. There is probably a shortage of parts at the moment too. 

In terms of brands I do like the idea of Nibe but they seem much more expensive and I wonder what you really get for that extra cash. 
I like that they are heat pump specialists and have been in business for a good amount of time. Being Scandinavian made also appeals to me.

I am wary of standard boiler manufacturers coming out with heat pumps. This is perhaps in justified. Honestly, I don’t know which brand would be the best balance of cost/performance/warrenty etc. What is your views on this? Most installers probably have a preferred brand and most customers probably prefer their own install i guess. 

I have an electrician coming round soon to look at a rewire so hopefully he will help with the DNO. I’ll attach a picture of the incomer here. Nothing on it to say what the rating currently is. 

As for the UFH, I do like our floorboards. To properly insulate the house I think these will have to be lifted anyway. I’m not totally against the rads idea though. The worry is that they will have to huge but maybe that is an old way of looking at it. 

@gordonville Both are good units, the Valiant controller is very complicated whilst the Nibe unit is much easier to operate and has a built-in online connected through Wi-Fi. I have installed both, but prefer the Nibe unit. (they are similar in price at source and both have excellent backup)

The installers should, according to MCS supply you with full room by room heat loss calculations with an emitter schedule (under floor or radiators) stating the flow temperature at the design temperature (around -3 or -4 for you), It must also include evidence that the heat pump will supply sufficient energy at this temperature without assistance as air source heat pumps loose performance and output at lower temperatures.

If the installers cannot supply this, look for another one.

Correctly sized radiators can supply excellent performance and comfort, but the design temperature should not be greater than 45 Deg C. Under floor should have a design flow temperature of 35 Deg C, a pipe spacing of 100mm usually suffices.

We are very busy at the moment, there is generally not a problem with supply, but people are tying to get the installations done before the RHI finishes in March next year, we have customers that we quoted 3 years ago now requesting installations.

I hope this helps.

@gordonville I posted the heat loss calculator and ASHP selector from freedom heat pumps on here a while ago. Have a look, it should help you in cross checking any quotes