ATCO here.
Can't understand why anyone is defending the BA pilot here, unless there had been some backstory that isn't on the youtube recording that subsequently influenced the exchange we heard.
The pilot should not have taken a line up if not ready. As ATC we couldn't care less whether you're not ready because of the cabin, final figures or whatever. As far as we're concerned, you can either takeoff when we ask or you're not. If you're not ready then tell us. Personally I will give you a little leeway but if you're starting to delay aircraft behind you by more than a couple of mins I'll do what the DUB ATCO tried to do and move you away from number 1 at the holding point.
And did I imagine it, but did I read somewhere that the aircraft then lined up and then caused 2 go-arounds by his delaying tactics?