How can one be 'based' in the UK unless one has the right to work/reside ? In days of gold, therein was the problem first. One of my dreams was to live and work as an airline pilot in the USA. Getting the right to live & work was the first, major hurdle. Same with Canada. Gave up.
Later, lived and worked with an Italian carrier 'based in Milan' on work/res permits and licence validation. Worked just fine. Just prior, in m y former company, Italian division required full UK ATPL writtens and type ratings to fly our UK registered aircraft out of the UK, just for extensive line/training. Politicians just trying to close up potential political loop-holes, I guess.Doesn't half create problems and divisions though.