yes - but I had planned on fitting a small pump / flow sensor if the
water pressure is low.

Yes, you wouldn't be able to have a shower easily. The shower head would be
above the water tank. This would cause you to need a negative head pump
(which is basically a kick start attachment), but you may still get airlock
problems. You really should consider replacing your hot water cylinder with
a heat bank or unvented cylinder, which will provide mains pressure hot
water (and run off your existing central heating system).

As for the radiators, there is another alternative to pressurising or moving
the header tank that may require less work. Basically, the new loft circuit
is done as a pressurised system with its own expansion tank, pressure
relief, filling loop, pump etc. It is then tied into the existing system
with a plate heat exchanger, which appears like a radiator to the existing
system and as a heating boiler to the loft system. If you get the controls
right, you can even have separate timing on the new circuit so you don't
have to heat it during the daytime. Andy Hall fitted a system with a similar
principle (but for different reasons, of course) for his shed.

Christian.