Hi Kokpit,
Flying over here in the sunny UK (yes it is very warm for the last couple of days - I brought the nice weather home with me from Florida on Sunday!) on an FAA PPL is perfectly possible.
Basically the United Kingdom CAA allow ICAO PPL holders (of which an FAA PPL is one) to fly G registered aeroplanes in United Kingdom Airspace in VFR conditions only. Thus if you have an FAA PPL/IR this would only be valid for VFR - ie your IR wouldn't be valid.
However if you source and buy an N registered aeroplane (non US citizens will need to do this on a trust company scheme) then you will be able to use your FAA PPL/IR in all airspace under VFR and IFR rules anywhere in the world and that includes Europe.
I've just got back from Florida where I obtained an FAA CPL/IR. I'm now trying to source an N registered aeroplane to hire so that I can use my IR on. I also have a CAA PPL/IMC/Night and currently fly a G registered aeroplane, but wouldn't mind being able to fly IFR in Europe - thus the need for the N registered aeroplane myself.
Good luck with your Instrument Rating... just remember enjoy the course and look forward to the flight test.
Hope this helps.
Best wishes,
Richard.