Second WestJet. Their 787s are very new, and the Economy cabin is as nice, if not nicer than AC's in my estimation.

We flew to LHR this May for something like CAD300 one-way. They often have prices in that range, or lower, although they can go significantly higher in summer proper. Looking right now, you can have a round trip from LGW to Calgary in June and returning June or July for around CAD800, or less on certain dates. Flying out of LHR is slightly more. Leaving in July is quite a bit more, and they don't seem to have anything for August yet.

It may be that you are looking too early. Give it a couple more months, and monitor WestJet's site for sales.