depends which airline - BA don't require passports for children for domestic flights, Easyjet don't either, but the check-in staff don't always know that (they've given me a hard time in the past), don't know about Ryanair. Flybe (who have taken over soem of the BA domestic routes) claimed not to need passports, but then changed their mind (DD2 was visiting my parents, passport was at home, she was flying back as an unaccompnied minor - fortunately no member of check-in staff is a match for my mother ).
Ring up the airline and get them to confirm to be on the safe side as policies change all the time.