You could look into getting a citizen card. I used to own one for buying alcohol in the days before I could grow a beard(!). Not sure how much they cost but heres a link;
CitizenCard Proof of Age - ID card, UK national ID Card, UK identity card, photo ID, citizen card, airline ID, proof of age, UK ID card, official ID, identity card, flight ID, YAP, flight identity, proof of identity citizencard, citizen card, photo I
Check the terms and conditions with the airline before booking tickets though. Easyjet and Flybe definately accept them as ID for internal travel but I'm not sure about other airlines (I'm pretty sure Ryanair insist on passports).
