If you can afford the money to fly BA then fly BA. I've used TS from Gatwick and their seat and food is shocking. As others have said, if things go sideways, you could find yourself very stuck.

Apart from the covid rules, as you're flying to see your girlfriend, if she is Canadian, immigration may be tougher as they may see you as a risk for overstaying. If you have some sort of evidence that anchors you to the UK, it might do you well to have a copy with you.