YANBU at all! My earliest flight was with DD1 to Miami when she was 12 weeks old and I was a gibbering mess so well done for asking for advice.
I have done LHR YYZ (Toronto) about a gazillion times with the DDs! Which airline are you flying with? And what time of day is your flight? And I am assuming it is just you and her? No other kids or grownups?
BA and Air Canada both have bassinets for babies but the weight restrictions vary. Try and book a bulkhead seat where they will put the bassinette on the wall in front of you so that you can put her down. With any luck she will snuggle down and sleep and you can watch a film or read or whatever. Yuo will have to have her on your lap with an infant belt for takeoff and landing though, and also if they switch the seatbelt sign on due to turbulence.
Take advantage of pre-boarding to get yourself in and settled before the throngs of other passengers.
Introduce yourself with a nice smile to the flight attendants at the beginning of the flight. The friendlier you are the more helpful they will be when you neeed it most! They will warm bottles, give you extra blankets and if you're lucky hold her for you while you go to the loo.
If she is restless you can let her play at your feet, but whatever you do, keep her out of the way of the trolleys. These things are seriously heavy and their brakes have been known to fail (my brother is a purser for Air Canada and tells some horrific stories).
Give her a bottle or a drink of water or something to chew on for takeoff and landing. The swallowing will help equalise the pressure in her ears and hopefully make her more comfortable.
Foldable McLaren or other umbrella-type stroller is a good idea through the airport. Try to get them to take it on board because you might not get it back in Toronto until you collect your bags off the belt.