Some British airlines insist on a UK-issued JAA licence. The reason is because there have been some fraudulently-issued licences from other JAA countries, and the CAA, for all its inadequacies, is at least trusted not to issue licences to people who don't qualify.
That has a knock-on effect in that I understand a UK licence can only be issued if you have attended a UK-approved and regulated school.
Scroggs