The prices of these flights seems excellent even after bag charges. So excellent I'm genuinelly amazed Air Canada think they can compete.

In Y for a range of dates seemingly throughout the entire year - even peak months like August - the typical fare is £380-£420 round trip from LGW to the West Coast of Canada. This is roughly HALF the cost of Y on Air Canada or British Airways.

How can AC be expecting people to pay double the cost of Westjet on, say London to Vancouver? It's not as if you get an experience thats twice as good, infact it could be argued that Economy on the 777HD is worse than economy on the 2-4-2 configured Westjet 767.

What's the catch? Surely Air Canada have to respond otherwise why would anyone bother with them?

Y is by its very nature price sensitive, I can think of zero typical Y travelers who would pay £400-£500 more for an economy ticket because it left from Heathrow instead of Gatwick. Anyone who can spend that much just for a trivial convenience factor is not buying discounted Y tickets anyway.