Under ICAO, no European country can stop somebody flying into and around Europe
Thankfully that is not true.
How do you think that certain airlines with poor standards can be banned from Europe?
Under ICAO, a country can require aircraft based in that cuntry to be placed on it's register and pilots flying it's aircraft in that country to be licensed by or validated by that country.
The US is a perfect example.........bring a G reg to the US with your UK PPL and if you want to keep it there you will have to both put the aircraft on the N register and obtain an FAA pilot certificate.
Regards,
DFC