The obvious legal position is that everything not prohibited is allowed
This should be obvious, because e.g. no law says you cannot fly wearing pink underpants, therefore you can.
So, if somebody says you cannot do XYZ, they need to provide a reference for that.
The FARs do not contain any known stuff banning the use of an FAA license in a non-N-reg plane. So that is allowed.
The UK CAA allows the use of any ICAO license for a G-reg. So, that's good
The only issue that I see is if the owner of the (non UK) airspace where you are flying objects to such a license / aircraft registry combination. The airspace owner is entitled to do this, because (fairly obviously) every country, ICAO member or not, has absolute jurisdiction within its frontiers. But I have never heard of any country thus objecting. It would make a mockery of its ICAO membership, for starters.
The above is for Europe. Most of the globe is what we call "3rd world", and most of it comprises of military dictatorships. You cannot just jump into your G-reg, with a UK PPL, thinking that is fine, and fly across Russia, Iran, China, Angola, Libya, etc. You need overflight / landing permissions, crew visas, etc etc etc. But that's a different topic. Once you get these permits, these countries still respect ICAO,
AFAIK. (well unless you happen to be a citizen of one, in which case they might grab you and make you do the military service
)