This is another point of confusion for myself.

To take the CH and/or UFH from the store or the boiler circuit.
Obviously, to take it from the store means instant heating, but
depletes our DHW source.

But the boiler is capable of replenishing at a far greater rate than the
radiators extract heat, so there is no massive effect. Just make sure that
store isn't already marginal in size.

Any other factors in deciding which side the heating should come from?

Direct heating has a considerable advantage when running with a condensing
boiler capable of open venting. You lose the advantages of sealed
pressurised operation, such as easy refilling, but you get a more efficient
system. The heatbank is designed to be heated to 75C. With direct heating,
with no heat exchanger differential to maintain, this allows a 75C flow
temp, rather than 82-85C. It also frequently provides a low return
temperature, with a nice large differential. This provides good condensing
action in the boiler.

Christian.