In the UK the revolving door for 737 drivers is fairly limited to FR, DY/ & J2 the people who join J2 are in the main doing so to be closer to where they live, the same with the many Captains joining FR this year, DY/ have not recruited in 2015 and for now can only offer LGW and even that is very unlikely for new starters in either seat.
People leaving any of the above for BA is a + career move, anything else is shuffling the cards