I fly this route two or three times a year in season. As has been posted, it's a nice short flight 4.5hrs E/B and a little over 5 W/B. It's an AC mainline aircraft with NA business class (no lie flat seats) but fine for such a short flight. Only downside is that it departs 11pm Nfld time and gets into LHR 6.30am UK (so 0300 Nfld) so with the best will in the world you are not getting much sleep. But for those of us travelling from YYT, it sure as hell beats flying to YHZ/YUL/YOW/YYZ first only to fly back over my house 10 hrs later.
It's usually the last flight out of YYT for the night - although the seasonal WJ YYT-DUB will depart after it next year - and there are few facilities at YYT as it's a small airport. But it also requires little time for check in and security compared to a larger airport which is a definate bonus. There is an AC Lounge airside but it's pretty small. It does stay open until the flight has departed.