I use kayak.co.uk for flights. It's never let me down! The +/- 3 days feature (it's a bit hidden, but it's there!) is really useful as it shows if you can make savings by changing your flying days slightly. Friday is not usually the cheapest day to fly, so if you can be a bit flexible, you might save. If you use 'LON' as the airport code, it picks up all London airports. The other think you want to check is legroom. It varies wildly and if you are going to be 8 hours on a flight, check you won't be too cramped, especially if it's an overnight flight. I'm 5'11" and legroom can be a real issue for me on longer flights.