For anyone else wondering, I was able to get out of Heathrow without a visa. I didn't even say anything. Just went to the officer, showed my passport and PR card, she asked a few questions like where I work, how I got my PR, what I plan on doing and when my departing flight was. She did seem a bit confused but asked her colleague who was seated next to her. Ultimately, she stamped my passport and let me go through. Process took 3 mins at most. Was able to get out, explore London and meet my friends after yearsTried this successfully at the start of this year. As others have said, with Canadian pr and departure and arrival on SAME day (not just <24 hours), you MAY be given a same day transit visa stamp without needing to apply for a specific visa before hand. But there are two caveats
1. This is purely at officers discretion. So you cannot plan for it, an officer might just refuse you and there is no way to dispute that.
2. This is meant for transit and cannot be used to say visit London attractions during a long layover.
So if you MUST exit airport, better get a visa beforehand. But if you are going to be in airport already and want to try your luck, ask an officer politely if you may exit airport for a transit hotel. And hope that officer is convinced.
I should have just trusted my gut and what was actually written on the UK Gov website instead of getting conflicting answers online.