Sounds like a perfect situation for a venturi shower. They work best
on mains pressure cold water and a gravity fed hot water tank. The
pressure from the mains effectively pulls the hot water from tanks
giving, allegedly power shower performance. Could be as cheap and
simpler than pump + valve. Google for venturi showers for more
information.

HTH

Peter

Thanks Peter - I forgot to mention, I had considered this option but
although I haven't measured it, her mains flow rate and pressure is
pretty poor and I read there is a fairly tight "window" of
compatability with Venturis in this respect. I will investigate further
though.
Luke