I've been on the A330 for TATL a lot more recently courtesy of flying out of YUL to avoid my seething hatred of YYZ connections, and while the A330 J pods are indeed smaller and have less storage I don't find it particularly bad or cramped (said as a 6'2" guy). I'd pick my flight on schedule more than anything else, especially since the YYZ-LHR segment is pretty short and it isn't like you are going to be in the seat that long anyway.