We have a Valliant Combi Boiler and I was wondering whether if I Tee off the 22mm Hot and Cold Pipes with 15mm Pipes for a Thermostatic Shower, then I take a 15mm Tee off the Cold Shower Pipe, would flushing the Toilet have any greater effect on the Shower than it would if we took the Toilet feed earlier on?

Also, what type of Shower should I be looking at? I want something that has very good performance (the Boiler does 16 Litres DHW) and is Modern looking, like the Hudson Reed stuff.