I think Dad would like a 'power shower' but believe you can't have
pumped shower and a combi / sealed boiler (if they were to go over to
one with a heating upgrade in the future)?

Although you can't pump a combi based shower, it doesn't matter as it
already has plenty of oomph. Choose a thermostatic mixer shower and an
external pump. When/if the combi arrives on the scene, the pump becomes eBay
fodder, but the flush mounted taseful chrome mixer can remain, along with
the tiling.

Christian.