BA and Air Canada for direct scheduled services, although AC also go indirect through eastern Canada and can sometimes be cheaper this way.
Another option is IcelandAir, which gives you the option of a nice Reykjavik stopover if you fancy it.
I've used Canadian Affair/Thomas Cook once before (Gatwick>Vancouver) and the service was great. All airlines have good & bad days, but the larger airlines will typically deal with the bad days better e.g.BA/AC have more planes available to replace flights or offer better compensation for delays. I think all the airlines are only legally required to compensate for lost baggage based on the weight (a few $ per kg), which is worthless. You need to claim on travel insurance to cover actual costs of lost luggage, in my personal experience.
Seattle is a good idea IF the savings are large, sometimes its just not worth the hassle. BA fly direct there from Heathrow. Seattle Airport>Vancouver is $100 return and a 5hr trip by shuttle bus. By car its a 2.5-3hr drive + border crossing time (anything from 5-60mins depending on time of day & traffic).