Xorthis,
Please correct me if I'm wrong
You are wrong.
You ICAO PPL is automatically "rendered valid" under the relevant section of the UK Air Navigation Order.
Your FAA licence is 100% suitable to rent a G registered aircraft in the United Kingdom.
Don't waste your time, effort or money on a JAA PPL.
By the way, you answered my previous comment yourself. Really bad idea to finish PPL then stay doing 100 hours plus of unsupervised "hour building" (during which you will teach yourself some very nasty bad habits and then ground yourself for six months doing the groundschool before attempting a very professional course for the CPL.
Save your flying until AFTER the exams are all finished - and even then, take some serious lessons to refresh yourself.
>^..^<