I think the UK has added another 'tax' to flights out of this country, so that adds to the fees. I overheard that on the Central line, so I have no idea if it is true! HA HA!
Anyway...
Try Kayak.com. They have a UK site. Try Opodo too.
We just booked flights for NYC (thank GAWD!!!!!!), and when we were looking, FOR ONE PERSON, in June was 420 rt, and July was 900 rt! More than DOUBLE!? I was shocked. Big difference! So, we are going sooner rather than later. We booked for June.
So, maybe it is just a more expensive travel month that you are looking at, because it is summer, and the students are off?