I think the single YEG-LHR daily flight is pretty full year round (I had to book it 6 months ahead the last time I used Aeroplan) so not exactly a lot of reward seats available. I tried to book it for a regular flight a month ago and it's more expensive than going through YYZ

On other hand YEG-YYZ to MUC or FRA seems to have space and discount J seats ( I got pretty good price J seats on YEG-YYZ-MUC and on to TXL ) So getting a aeroplan reward seat is easier