A normal 100amp single phase connection is more than fine for a heat pump and solar, and a even a couple of EVs if you choose to (most modern EVs can only charge up to a max of 11 or 12kw on AC charging - most home chargers only provide AC-charging and the biggest I have seen is 7.4kw so not far off). We have all of these and very rarely get to even a fraction of even half of what our mains fuse capacity is (and that would only be on xmas day when everything is running and it is sub-zero outside - which is very rare and even then not even close to our limit and most heat pumps run at a lot less than their advertised size). The size of solar inverter shouldn't be an issue if it is a "hybrid" inverter which will be DC-coupled. The only thing I can think of is they want to make sure any excess solar generated that is sent back to the grid can be coped with - however a single phase hybrid inverter is normally only sized up to a certain size anyway so all single phase connections should be able to cope (the biggest solar hybrid inverter I have ever seen on single phase is 16kw - but generally they are all around 8kw or less). We have a 7.5kw single phase hybrid solar inverter with whole house backup and large battery storage, air source heat pump and EV. We run our whole house including running the heat pump 24 hours a day off our solar batteries (which charge up from the mains grid during the winter at off-peak tariff prices and from the sun at all other times of the year). We have 21kwh usable storage. The sizing of our solar inverter was something I researched a lot and I sourced all of my solar equipment myself with a trade account I have, and had our electrician fit it all. I sized ours according to what our peak loads in our house might potentially be, as my main objective was to be less reliant on the grid and run entirely off our battery storage. I had to fill in a G99 application form (never done one before but was fairly straight forward) as my potential hybrid inverter was going to be over 3.68kw. And the DNO approved us for a limit of 8kw export, meaning my 7.5kw inverter was allowed. You have to get G99 approval first before anything is installed over 3.68kw (unless the law has changed since I did my application).
Each solar inverter will have something called a maximum continuous discharge rate, which ultimately tells you how much load in your house it can handle before the grid cuts in. Normally the smaller the advertised size of the inverter the smaller the max continuous discharge rate - the discharge rate is not always the same as the advertised "size" of the inverter (it is sometimes smaller) and it is not always the "max continuous rate" (as it may only last for 10 seconds. We have the Solax single phase hybrid inverter which allows a maximum continuous discharge rate of 7.5kw - you also need enough battery storage to get to the max potential discharge rate. You have to look in the specifications or small print to find the discharge rate but that is ultimately how I sized and sourced my inverter (a professional installer will normally do this all for you) - as well as being able to handle the size and number of PV panels on our roof - all of this you are very unlikely to know at the point of building your house. It is a bit different when you come to older more traditional solar inverters which are AC-coupled single phase string inverters (an example of this is a string inverter connected with a Tesla Powerwall) where the battery storage is also exposed to the grid. But again all of this can and should be able to be handled by all modern 100amp fuses and cabling to the local transformer. One thing the DNO may try and do is quote you for a three phase connection which is often an astronomical price compared to the more common 100a single phase connections (hence why they like to quote it as it is a lot more money for them). We had a new 100a single phase connection put in for our self-build and it was about £6k, which included all new cabling to the nearest transformer (I did all the labour for the trenches for the cables) and the new cabling is also three phase compatible so if we ever wanted three phase in the future it is only the transformer which would need replacing. But three phase was going to be over £12k total to get and not needed as we don't run heavy load any workshop tools etc. This ended up being quite a long post. In summary, I don't think I would really know what size solar inverter you are likely to want at this stage but I also don't think it will have much of a bearing on your connection (as described). In my opinion the regular 100amp single phase is far more than ample for most modern fully electric houses (like ours) but three phase is great if the quote is reasonable, so as to future proof any needs in the future.