You obviously mean just for a shower as a tank fed bath can supply very
high
flow rates, but poor pressure.

That would depend on the pipe diameter and routing. Most gravity fed baths
I've used have had substandard flow rates, sometimes not much more than a
combi boiler. A pump is often quite an improvement. The problems occur when
the hot water cylinder is tucked away somewhere quite different from the
location of the bath and the cold water tank, leading to convoluted pipe
runs, sometimes even run in 15mm pipe. It is these situations that a pump is
often quite an improvement as far as baths are concerned. With showers, you
usually need a pump to improve the pressure so that the water can squeeze
through the holes.

Christian.