BTW, I'd avoid the traditional thermal store type devices. I would only
choose a model with a plate heat exchanger for DHW operation.

Why? I was thinking of getting one of these (Albion mainsflow)
because of their simplicity.

Advantages:
1. Simplicity.
2. No electrical power.

Disadvantages:
1. Reduced pressure (may not matter, depending on your supply).
2. Water temperature variable with flow (can be fixed with external TMV).
3. Lower maximum flow rate before temp drops too low.
4. Internal heat exchanger makes descaling difficult and replacement
impossible.
5. Worse destratification compared to external loop, leads to more gradual
temp drop over long draw off, rather than remaining hot until sharper cut
off.

Christian.