Boosting the shower pressure.

The pressure from the mixer tap weak.     I live in a converted flat where access to the cold water storage tank is in the neighbours flat above me.     The hot water cylinder is in the kitchen and space is extremely tight to fit an impeller pump.     Would it be foolish have pump under bath? There is an electric towel heater rail in there already in the bathroom so was hoping to feed from there. I was thinking directly to the pipes serving the bath taps as a in/out to the pump. Where in London should I get the parts at a reasonable price?
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