Our water pressure is not that high, but I do seem to have good flow
(20.8 lpm at the kitchen tap if what I just measured is right).

It is acceptable, but not good. However, kitchen taps aren't all they used
to be. Modern designs are quite restrictive and in some cases, the total
flow available to your house will actually be higher.

I would certainly not hesitate to use this flow rate for a combi boiler, if
one was suitable for the domestic requirements. However, I would not spend
money on an unvented cylinder or heat bank, as the full benefit would not be
obtained without the addition of an accumulator.

Christian.