I'm aware of all the hoops you have to go through with the CAA for foreign registered aircraft - used to know a Ukrainian-registered and crewed Turbolet operating in the summertime for a skydiving club in the UK.
Meanwhile The Irish IAA are ultra-bend over backwards to help. Following one recent proposal of theirs Ryanair said they would re-register all their fleet in Lithuania. End of proposal.
Another country which offers a lot of help to overseas-operated aircraft is France, which will do this for any Airbus aircraft needed to operate in a difficult territory (yes, Airbus is theoretically a European rather than a French organisation, but I am sure we all know the truth). There has always been separate French registrations for their overseas territories in the F-O... series, you will see many Airbus aircraft worldwide with these.