Mike
Couple of points - does depend on where you're going but if the guy is currently in the UK he is 3000+ miles from a field office.
200 miles is not a long way to go - maybe the FBO will have an airplane and maybe the chap will want to get a check out and, who knows, maybe want some real experience of flying cross country - overall I would think that having the field office a couple of hundred miles always is a plus
No the activation is not a red herring - it is an absolute requirement - been there done that - everyone exercising the privileges of an FAA license MUST have a flight review from an FAA CFI within the previous two years - the EASA flight review does not satisfy that requirement.
In addition the originating licence MUST be valid i.e. not have been revoked or suspended, have the appropriate medical and flight review from the originating ICAO country - yes, even to fly in the US.