They used to do a LAMP at Heathrow for departure which I would use for Canada molecular, but I think they've stopped now (I guess not enough demand). When I went to YVR last week, I used the Rapid PCR at Collinson which comes back with a result in 3 hours (NOT the regular RT-PCR, which is of course acceptable for Canada, but it just takes longer to get a result) together with a BA code (so 76GBP total) (even though I've never flown BA; they don't check) . However, they are offsite - they do run a shuttle bus there now; but maybe you might prefer to use ExpressTest's Rapid PCR which is located at the airport, but it is more expensive. Depending on the time of your flight, you might even be able to do the testing the morning of your flight, or if not, the previous evening. I've always had the results back in the time they promised.