In the UK there is the rule or 2 out of 3.
The 3 factors are aircraft country of registration, pilot country of certification and place where flight takes place. Any 2 of those must be the same for the flight to be legal.
A US pilot certificate holder may fly a US registered airplane anywhere.
A British pilot may fly an aeroplane registered in any country in the UK.
A pilot from anywhere may fly a British registered aeroplane in the UK.
The first is obvious, I've seen the 2nd happen a lot, the 3rd is a little less common and I'm not certain is legal. I don't know if this holds true in the rest of EASA land, but would be really helpful for Robin if it was.