Havick - thanks for that. I agree that, in reality, the only sensible option is probably to get a full FAA license.
Gordy, thanks for trying to help but we seem to keep talking past each other. Your quotes clearly show that the FAA 61.75 certificate is subject to the limitations and restrictions of the foreign (UK) license, but I cannot see where it says that it is also subject to the same ratings and privileges and I continue to believe that these are two separate sets of things. Thanks for your help in any case.