Stephen, your system does sound well tuned to how you want it to work. If that works, great. Can’t knock it
My experiences are coloured by the daily support calls I get where systems are not plumbed up correctly, the control strategy is not there etc. All the systems I work with keep the potable DHW completely seperate from heating circuit water via a cylinder coil or PHX. Whilst I will not confess to know the water regulations here in the UK in full, I do know that, generally speaking, we do not allow the two to meet.
edit - Since looking up water regulations in the UK, Wholesome mains cold water is fluid category 1. Stored DHW is fluid category 2. Central heating water for space heating is fluid category 3. Backflow prevention / seperation of them is required via non return valve or air gap depending on category & situation. In other words, in the UK, I would personally not share UFH water with the DHW tank directly.
There are also legionella prevention concerns sharing heating water running on UFH with the DHW tank. Your typical UFH circuit will be operating at a nice low temperature (30C?) which is an ideal breeding ground for legionella. The heat pump could be heating the DHW tank to 45C daily and once a week 60C to kill off the bugs. If solar is in play, it may be heated to 60C daily, but not necesarrily in winter months in the UK. The water in the UFH would not be heated to 60C however, and legionella could continue to grow there, then get pumped around to the DHW cylinder.
I know a service engineer who has caught Legionaires and thankfully lived to tell the tale. It’s not pretty.
Going back to a solution for Pete. It might be possible to share the same DHW cylinder coil between heat pump and solar via a 3-port valve (and 2-port shut off for G3 if applicable) if the heat pump is told not to service DHW when the solar is running. My concerns is if the heat pump is flooded with excess heat on its return pipe due to the heat from the solar, that it will either shut off prematurely whilst servicing UFH or hit high pressure fault. We still want to be able to service UFH via heat pump regardless of what the solar is doing.
As Bosch Worcester also make solar thermal panels, its likely they’ll have a hydraulic they’ve tested as good to integrate the two, to save Pete pulling his heating system apart as an experiment just before winter arrives here.