Where can I get cheap flights to Toronto, Canada from London?

I have looked everywhere and the cheapest I can find is around �400. I want to go with my granddad (so there will be two of us) and we want to stay for three weeks, which means we want a return flight. We want to go at the end of June or the start of July. We'd like July better (even if it was a week into the month) but I don't know if that's the holidays so perhaps the flights will be lots of money then?! Anyway, I don't know what to do, but we want a flight around �400. Cheaper than that would be better!!! We will fly from any airport in London, UK.