I think you'll find that only A,B,C and D are regarded as Controlled Airspace. 'B' doesn't exist in UK as far as I am aware. 'F' and 'G' are open FIR and the privileges of the FAA License/IR could be exercised there but not for instruction and not for hire or reward.
I'm not sure why this agreement exists but it seems to point to the private user with the right tickets having the ability to get around the UK in a 'G' registered aircraft and, allowing the FAA Licence holder living in the UK to maintain a level of proficiency on instruments.
Do remember, of course, the FAA licence, medical and IR must all be current and must not restrict this type of flight.
Is there something specific you want to do or is this just a point of interest?