i have lived in London for over 20 years now. i remember when i first came here the return flights home used to vary between £69 and £120 ish but saying that the Xmas flights used to be full of boozy irish guys and gals and demand was generally high all year round. now i pay at the most £150-£200 at the most for the 4 of us return in school holiday breaks so I am not complaining.
the way to keep it cheap is, hand luggage only, check in on line, pay for tickets with an electron card (which I am going to apply for next week at the Coop Bank in Wood Green) so hopfully no more credit card charges next time round and car hire is still good too £250 for two weeks for a Golf or similar.