I thought I'd drop back into this thread and just update my experience with the Westjet connection.
I was able to check in online for both flights but I couldn't get a boarding pass for the Westjet flight. Easily sorted at Heathrow though. It didn't seem to be possible to pay for seat selection but I had row 6 on the short Westjet flight which seemed to have slightly more legroom. Because I had an overnight in Vancouver I was able to get my checked bag back at Vancouver (but it could have been checked through). The following morning at Vancouver airport I was quickly through to the gate. No lounge access but Tim Horton's supplied me with breakfast for just a few dollars. Boarding was strictly by rows from the back of the plane and there was room to put my hand luggage up overhead. On board in Economy there's just a soft drink on offer which is fine for a flight of around an hour.
On the way back from Edmonton I got the boarding pass for the Westjet flight at checkin. I decided to check my bag in even though it could have gone hand baggage (guess whose bag was out nearly last at Heathrow?). The flight was nowhere near full so there was a spare middle seat in most rows. It was a brand new plane (737-8 ?) with seat back TV screens even in Economy playing a range of TV channels. Changed terminals in Vancouver and headed to the 1st lounge which, quite honestly, was a bit of a let down food wise. Would much rather have tried the Cathay noodle bar but the lounge doesn't open until 2000.
Overall, perfectly acceptable trip on Westjet. No TPs or Avios and the flights don't even show on the BAEC activity page.
The only negative part of the whole trip was the terrible IFE on board the BA 747 going out. Very few new movies so I thought I'd try the "movie library" - which contained a total of 4 films. Not a very big library then. On the way back the 747 had some new LCD screens with a much better selection. I must admit I'd missed the fact that there was a variety of IFE options on 747s. Will have to check more closely next time.