@Star Cruiser, let us know the feed back from ACV when you get it.
Your general spec looks a good one. We are seeing more and more larger properties run hot water circulation loops on thermal stores - it's a standard spec for large property new build or re-furbs. so it's not unusual. Tee off should be kept as shorter as possible, else the purpose is lost (they just become dead / stagnant ends even possibility of legionella..) make sure both the flows and returns are well insulated, else it becomes a secondary heating system
Properly sized thermal store should cope no problems, and they can be vented or unvented.
Your Solar thermal may be a bit undersized, if you've got the roof space you could consider going for the 30x58.
ImmerSun units are excellent - go for two x 3kW immersions, one high and one low and use the change over relay in the ImmerSun for when the top is hot (we recently did a 2500 ltre unvented thermal store with 3 ImmerSuns and 18kW of immersions
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Your hot water is then supplied either by a built in coil in the thermal store, or it can be done by an external plate heat exchanger (there are lots of advantages of that approach).
Make sure you've got plenty of connections to put heat into the thermal store - natural gas is a cheap way to heat, you may wish later on to add in say a wood burning back boiler stove or an air source or ground source heat pump. It doesn't cost a lot now to add in extra connections / coils and gives you options later.
Remember that as a self builder, you'll qualify for the RHI on certain elements as well
Sizing thermal stores is a complicated business!
To be done properly it needs to be based on full heat load and demand calcs (heating and hot water - it's not just how much, it's also when)- whoever has done the ufhs design should have full room by room power and energy requirements, - that will give you your peak power needed from the 'boiler' and thermal store - stores are often used to meet a very short term peak demand that can't be met by a heat pump - you can get 12 kW peak out of a 10kW heat pump with the right sized thermal store - it just depends how long you need that 12kW for
Make sure the boiler controls are suitable for interlocking with the solar thermal - you'll want to give that a chance to do some good before the boiler tries to over power it! Also make sure you've got weather compensation controls on the heating system as well as properly designed ufhs controls, a properly designed thermal store system will also help to ensure that the condensing gas boiler also actually condenses.
There are a few good people up in your neck of the woods that I know that would be interested in quoting for you - drop me a pm if interested.
Looks like a good starting spec, - Have Fun!
You've got the details for ACV, so here's another
:: AKVA SOLAR
Couple of images attached illustrating sizing thermal stores: