Another potential option I've just spotted is to go via Xiamen in China. Cebu Pacific are doing a promo in March from Xiamen to Manila one way for 88 pesos plus taxes & baggage. Although the offer expired a few days ago it will run until the last seat is sold. You might get lucky.

Flights to Xiamen from Heathrow seem to be around £480-490.

Had a look for different dates in March for flights direct to Manila from Heathrow and I haven't seen any for less than £600.