mapleworth, Genghis the Engineer has provided a lucid and accurate explanation. The FAA do not grant an approval to a non FAA pilot to fly an 'N' reg aeroplane in a single country (state in formal parlance) only. If you are granted such a right by any state then it applies to the world. No 3rd party country has a say in it.
It is true to say that the licence standards throughout the world are not exactly the same. ICAO only provides for a common minimum and acceptable standard. ICAO doesn't micro manage in the same way as EASA. Many countries add various additional elements to their own issued licences as they do with aircraft certification and the maintenance schedules. Your EASA licence is an ICAO one but with EASA bits and pieces added. The same goes for aircraft certification and the maintenance schedule. That does not affect the rights of a foreign registered aircraft being flown by their approved pilot, who should not be obstructed. A state does have the right to prevent a foreign registered aeroplane from being permanently based. Most will accept such an aircraft to be based for a short period of time whatever it decides. The UK does not set a period that a foreign registered aircraft may remain. That is why you will see so many USA owned and registered aeroplanes based permanently in the UK.
Mapleworth, to put things in a nutshell your spending too much effort in suppositions and then looking for the verifications that it is not possible to find. As for EASA and the UK, the CAA/EASA are not giving any hints to any agreements that may follow after January 1st. Your contact at the DGAC has it all in perspective, the gallic shrug is well placed in all this, be assured.
Last edited by Fl1ingfrog; 14th Oct 2020 at 21:00.