I'd be totally amazed if you could sign somebody off to solo in the UK if you only had an FAA certificate. Check the FAA web site for info on trainings schools/examiners. There is indeed one FAA examiner in the UK, must be recent, there wasn't one when I last looked. There is also a Part 141 school outside the US, so it's possible, but the one I found is in Amman, so might not be there for much longer.