Just to let everyone know that after the London bombings you may now be asked about your immigration status when checking in for internal flights in the UK. This happened to an American friend who has just visited Northern Ireland from Bournemouth.
Since she doesn't have a British passport and her English driver's license doesn't have a picture, she always uses her US passport as photo ID. This time, for the first time ever, she was asked to clarify her immigration status at check in at both airports. Her ILR is stamped in the passport, so there wasn't a problem, but you might want to be sure that you're carrying appropriate documentation. I don't know if I would have had a problem with my American accent and photo Northern Ireland license.