As others have said, there is no requirement for id checks on internal UK flights. Although Gordon et al want to introduce this. With bmi as long as you have either your booking ref, credit/debit card, frequent flyer card or passport you can use the self service machines to check in. Even if you have your passport it won't necessarily be checked by a human, just the machine's scanner. I always check id if it is proffered, but if not then as I said before it is not a requirement. Letters to "What, trust you with my personal details?. 10 Downing Street, London W1."