dont know if these can be used on a combi boiler, but I wanted to fit a shower without a pumped hot water system.
I was recommended a 'Trevi' 'Boost', this uses mains pressure cold feed and with a fancy bit of magic inside it, using the cold preasure it gives a boosted hot. Dont know how it does it, but it works.
Costs a little bit more than a standard mixer shower but well worth it.
Got mine from Jewsons
http://www.trevishowers.co.uk/treviboost.asp