Shower feed pressure problems

Hi, i'am looking for some advice or any comments on what should be done...
We'd like to put the best shower(powerful) into an old house terraced however the cold water feed to the bathroom is via a tank in the loft and we're on the ground floor, with a sperate flat above us. The hot water is mains fed via a new condensing boiler capable of heating 17L/min - so the pressure between the two is pretty different.
Can anybody advise on a) do we need to balance the pressure or will a thermostatic valve do that for us? and if so b) what would the best way of doing this - without affecting the flat above us?
Thanks for any comments or suggestions on this one.
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