Good name!
Assuming that this is a gravity fed system - that is, the tank in the loft is a large tank fed from the mains, which feeds the cylinder which then supplies the taps - then you will have lower water pressure in your proposed ensuite than you do on the other floors.
Whether the difference in flow rate is big enough to affect useability depends on a number of factors, including pipe sizing, exact tank height above loft floor and the tupe of fittings you intend to use.
I'd be inclined to hook up a garden hose to the hot tap on the first floor, run it up the stairs to the second floor, and measure the flor rate (hint - you need a bucket and a stop-watch). This will give you a worst-case flow rate which you can improve upon.