You need to find an FAA Instructor and a N Reg machine, or a FAA & JAA Instructor in a JAA reg. machine, do 5 hours dual (as you already have your R22 experience if you got your ppl in it), performing the requisite procedures in SFAR 73 to Part 61 of the Federal Aviation Regulations.
The abnormal and emergency procedures that need to be covered are
(i) Enhanced training in autorotation procedures,
(ii) Engine rotor RPM control without the use of the governor,
(iii) Low rotor RPM recognition and recovery, and
(iv) Effects of low G maneuvers and proper recovery procedures
Of course, this will only allow you to fly N-Reg R44's unless you have another license with appropriate type ratings /endorsements.
An interesting tidbit, is that SFAR 73 expires June 30, 2009. I am not aware of it having been renewed. If it does not get renewed then all you need to do is wait until July, get someone to sign you out and Bob's your uncle, though for your own safety and that of those with or around you I would suggest getting checked on the above procedures regardless.
The above is covered on pages 33-35 in my 2009 FAR/AIM.
When in doubt contact the FAA, they are pretty helpful (have been to me anyway) - They might even have representation at the London Embassy.