Afraid not. I was the opposite side of the aisle, and boarded before him. His seat belt remained unfastened the entire flight, and for landing, and as I say, he yacked on his phone throughout the demo.
When the crew member did her cabin secure, it would have been obvious the seat belt was dangling down the side of the chair into the aisle. But anyway, I know what it's like, we ask somebody to comply with the cabin secure, then get abuse. Rarely a flight goes by without it happening. If they don't comply with the request, the FSM/CSD cathes us out, we are in deep doo-doo. It doesn't matter that they buckle up when we ask, if they unbuckle when we sit down, then we still get it in the neck. Trust me. Aye, it's a glam job
I don't think you work for BA from your post because we don't work like that, no crew member in BA would be told off for a passenger undoing their seatbelt after the cabin was secured.
This really is going off topic and has absolutely nothing to do with what the OP said.
I did like your video though!