@ltron2 

The EPC on our house is A-92, however that is because we have a 6.6kW of solar PV on a south-facing roof. Without the solar, it is C-77, so very similar to your C-76 pre triple glazing. We have double glazing: Planitherm One, which has a U-value of 1.0 and a g-value of 0.49, which is close to typical triple glazing values. The A2A is two Daikin single-split systems 2.5kW and 3.5kW. In our case, two single splits were possible (detached house so two heat pumps are permitted development) and quite a bit cheaper than a multi-split (economies of scale and multi-splits are more complex). Single-splits have a marginal efficiency advantage as the indoor and outdoor units are perfectly matched and they modulate down to lower levels (about 125W input).

It is well worth while taking a few minutes to have a careful read of the relevant legislation for the BUS grant see: https://www.legislation.gov.uk/uksi/2022/565/regulation/9 in particular regulation 9(2) 

(b)it is capable of meeting the full space heating and hot water heating demands of that property, whether alone or in combination with supplementary appliances that are not biomass boilers,]

(ba)it has sufficient kilowatt thermal capacity to meet the full space heating demand for the eligible property alone, whether or not it is installed alongside a supplementary appliance, and

(c)it replaces the heat generating components of the original heating system installed in that property (where applicable), other than any supplementary appliance.

From my understanding, there are three ways that you could proceed:

1. With things as they are get quotes for an A2A system. You could look at something like a Daikin Multi plus that does both A2A and hot water. https://www.daikin.co.uk/en_gb/residential/products-and-advice/product-categories/air-conditioners/multi-plus-split.html With this approach, you would require an A2A indoor unit in each habitable room, which may be far more than you really want / need, and expensive. You can however get the BUS grant to cover some of the costs. 

2. You could (and you'd need to check this out carefully, not just act on my suggestion) install some supplementary appliances, for example direct electric heating in some rooms e.g. heated towel rail, electric radiator (fixed not portable), etc. You might even have an electric immersion heater if you have a hot water tank. These things are electrical "supplementary appliances". With these in place, a friendly installer may not need to quote for putting A2A in every habitable room or add anything for hot water. Provided the supplementary appliances can heat the rooms they are in sufficiently well, provide hot water etc. then you can perhaps get a system with just two A2A indoor units and still get the BUS grant. Note the grant comes to the installer and would be taken from them if the installation is not compliant, so they need to be confident that what they do in addition to any supplementary appliances meets the regulations. Note it will be easy to cover the full space heating demand, since that is not going to exceed 5kW for your property.

3. Forget the grant and do exactly what you want / need. A single-split 3.5kW system may well be sufficient. The disadvantage is no grant. The advantage is low cost and no hoops to jump through. You have choice of very well established A2A installers, who may not necessarily have MCS certification (it is no guarantee of quality and costs a lot, part of which is passed on to you the consumer).

Your triple glazing's relatively low g-value will reduce the solar gain considerably. If you are also prepared to use blackout blinds / close curtains to keep a lot of the solar gain at bay on really sunny days, then you will likely be just fine with the A2A capacity matching the max heat loss. That is where we are, heat loss is around 6 to 6.5kW and the A2A can do 6kW, in practice it's plenty.

Note I have no relationship with Daikin, I just have two of their A2A units which have worked extremely well over the past year.

If you've got a sash window or a door you can remove the plastic panel from and rearrange with the external unit outside cut a slot and then put back and insulate a bit then you could just get a 4kW Midea portasplit and just DIY for a grand or so.

It'll probably cover all your cooling and most of your heating but that'll depend on the building insulation, windows, leaks etc and size of course.

The big problem is that the two pieces can't be separated. With something like a sash window it works great as you just open the sash, put the external unit out and then lower it down to hose level. Couple of pieces of celotex cut each side and a bit of sheepswool for any gaps and it's cosy. Whereas with tilt and turn it's a bit of a mess and the hose kits for tilt and turn windows really only work for cooling.