Be careful. It is a requirement for use of the FAA licence outsie the USA that it is accompanied by FCC Form 605 FRC. Without this, the licence is not valid and technically you would not be flying with an ICAO Annex 1 licence.
Is anyone going to get excited? Probably not..........except the insurance company when the wheel comes off and they have a get out clause for not paying out on the policy. 'Sorry, pal, it was your responsibility to ensure that the person you hired the aircraft to was qualified to operate the aircraft LEGALLY'. The phrase 'opening a can of worms' springs to mind.
It only takes about 10 days (if that!) and it can all be done on line so there's no excuse for not getting one. It's a load of blocks, admittedly, but don't let yourself get caught out. It's not worth it.
And once you have it, you are authorised to operate RT equipment in UK registered aircraft.