I guess it really depends upon your own priorities. Do you look for the dirt cheapest price, or are you interested in comfy seats or are you looking for in flight entertainment. Food may also be important. Despite what some people say,there are differences in levels of service even within economy. Have a look at each airlines website to check on food, seating and general service levels. Also consider the flight timings, do you want to leave early or late in the day.
Are you connecting from a regional flight to London, as you may want to look at the flight connectivity, as not all airlines will check your baggage all the way through.
I'd say BA, but the legroom in economy on BA (31") is pretty poor, and may be better on Air Canada (32-34"), if you travel on one of their airbus flights.
For seating, have a look at seatguru.com
That's my thruppence worth.