You sometimes get asked for ID when checking in luggage.
I've printed off the advice on ba.com questions and it lists driving licence, passport or BA Executive Card. This latter was important to me as a very unpleasant member of the LGW team* insisted the BA card meant nothing and it had to be a photo-ID hence I keep the printed sheet with me.
Specific text reads:
We do not require to see a passport or photo identification if you are travelling on purely UK domestic flights (i.e. with no onwards international flights).
If you are travelling on an e-ticket, you will need to have the credit/debit card originally used to pay for the booking. This is for identification purposes only and you will not be charged again.
Travel agents' system seem to print all sorts of irrelevant and incorrect rubbish!
(*I should add, the individual at LGW is very exceptional; they are a fantastic bunch there normally)