This is how I understand the rules:
Hours gained under an ICAO licence count towards the experience requirements. However these hours must still be gained in a context that the CAA would regard as P1.
To find out what the CAA regard as P1 look at LASORS Section A "guide to logbook annotation". I don't have the link to LASROS to hand but do a search on PPRUNE and I'm sure you'll find it.
In short, the CAA will allow you to build hours on another ICAO licence, but they'll still decide how many of those hours count.
Hope this helps,
Toby