Been there, done that, useless. OK daughter in Turkey it worked, father-in-law North Wales complete failure, it was a con, they removed immersion heater so electric cost went down, it was only when the smart meter was fitted and pilot flame went out, realised all hot water was coming from gas not solar, we had people back to try and get it to work, this was the summer, they failed, father-in-law died and house sold it never did work. Looked the part, but did not work.
So bad install, that doesn't mean an install can't be done right - and as you've admitted, you've seen one that does work. And I can remember that something like 40 years ago (so when this was still pretty new stuff in the UK domestic market) we had a neighbour who fitted solar thermal and was very happy with it. He was an engineer, so you can be sure that he'd have been taking readings, drawing graphs (for the youngsters, we used to have paper with a grid on where we'd draw graphs with pencils and pens), and so on.
Only option is electric solar panels and excess fed to immersion heater.
These days I'd probably be leaning that way. One of the big problems with solar thermal is that you gets (if the system works) loads of hot water in the summer when we don't need much, and little in winter when we do need it. So the collector system needs to be sized conservatively to avoid needing large heat dump radiators.
With PV, as you say, you can use that to run an immersion heater (I do wonder about the relative efficiency), and there's equipment to automatically manage that. I wonder if there's an inverter with a variable voltage output so that it can maximise the immersion power while maintaining zero import or export during those periods when the panels produce less than the immersion plus house loads are using ?
There seems to be some uncertainty as to heat pumps and temperatures for domestic hot water
There's no uncertainty there at all. Heat pumps work best at low flow temperatures, and just from a thermodynamic PoV, it's not easy/efficient getting "hot enough" hot water - though I've not had reason to look at that part of the performance tables in the past so I can't be sure whether they can achieve it or not.
The minimum temperature for the DHW cylinder is (from memory) 50˚C, any less and you can't be sure of killing any legionella bacteria in a timely manner. But at the same time, maximum safe temperature (to avoid scalding) from a hot tap is (again from memory) something like 40˚C which is why you'll find TMV (thermostatic mixing valves) under every sink in places with vulnerable people
both gas and heat pumps want the radiators cooler for better efficiency
Which is why I suspect you'll find that many condensing boilers don't condense much (if at all) - especially retrofit's to older systems with lower output radiators.
which means either reduce losses, or increase radiator sizes, and we seem to be leaning towards only heat when required, so reducing looses will not help with reheat times, so with my house to move to heat pumps means a major job, remove the ceilings to install new pipework as the micro bore will not allow enough flow with lower temperatures.
If the designer knows his stuff*, the microbore shouldn't be a problem. But there is a tradeoff between radiator size, achievable flow rates, and flow temperatures. It might mean slightly larger rads removing more heat from a limited flow rate before it goes back to the heat source.
In our house, I've been fitting larger rads as I get round each room "doing up", and except for one room, even in the depths of winter it can keep the room warm at 40˚C flow temperature, and with the flow rate fairly well throttled.
* Yeah, I know, this is an industry where many of the installers arrive by horse and wear a stetson
But the major point is with a 60 amp supply there is no way we can have heat pumps and EV charging, if you look at most boilers they exceed 20 kW which is more than the total supply to house, at 2/3 of that, allowing for a heat pump to be more efficient that's the total supply to the house for heating alone, it is a three story house, so it's a non starter.
Boilers are generally grossly oversized in a domestic environment - especially combi boilers which have to be well oversized to provide a lacklustre DHW flow rate. Obviously a larger property would need more heating, but our rental flat can be kept toasty with an average of only 2kW in very cold weather - I fitted the thermal store during that very cold spell in Dec 2010, and measured the heating load by running it on the immersion heater for a few days. But for anything better that a dribble, the boiler needs to be around 30kW for DHW. And there are practical limits to how low a boiler can modulate - just shy of 10kW for the boiler in the flat. By the time you are needing significantly more than 10kW for heating in mild weather, you probably need something more than a 30kW (combi) boiler for DHW.
<rant mode>And, a personal opinion here, the whole heating equipment industry (at least, pretty well all of the domestic sector) is barely towards the end of the 20th century in designs. Almost all boilers can't handle more than 20˚C delta-T. So instead, we have systems where we feed hot water directly back into the return, reducing efficiency, and in some cases, preventing condensing. Not to mention the average new build house where they'll have put in (if they could be bothered and didn't leave rolls still wrapped up in the attic) the minimum insulation that meets the regs, and fitted the smallest radiators they thought they could get away with.
Your assessment of the supply situation probably isn't far off. At least modern kit has the option of a CT clamp on the meter tails so it can control demand - so that means your car would charge slowly in cold weather. But it doesn't do much for diversity assumptions the DNOs would have made when sizing the street cables, substation fuses etc.
