Under no circumstances can a power shower be fed from a combination boiler.
This is because power showers incorporate a pump, and the W.R.A.S. (Water Regulations Advisory Scheme) dictates that mains water which is what feeds a combination boiler cannot be pumped.
Contravention of this water regulation carries a substantial fine. Power showers can only be installed on a conventional gravity fed plumbing system, comprising of a cold water storage cistern and a hot water cylinder.