You've put your dates as March to April 2011, I'm assuming you mean 2012?
You should take into account that your intended dates of travel take in the Easter period, and airlines always bump up prices for that period. Anything less than £600 over Easter is a good deal. From painful experience of dealing with Saudi and having my flights cancelled/rescheduled on 2 separate occasions, I would avoid the cheapest fares with the crappiest airlines and pay more to get a decent airline - You will be fine with Cathay, Emirates, Etihad, Qatar, Singapore, KLM (if they still fly when you go).
The £580 deal Dedworth found with Cathay sounds like a good deal, although I paid £515 with Cathay to fly over Easter 2011, and have paid £430 to fly with Lufthansa to Singapore over Easter 2012.
Another option is to get a flight to Singapore or Hong Kong and a budget airline connection to Manila, this is always my preferred choice as it gives me the opportunity of a stop over on my outbound and inbound journeys. Flights to SIN or HKG are usually cheaper than to MNL as there is far more competition. Budget between £450 - £500 to get from LHR to SIN/HKG and £70 - £100 onwards to MNL.
