UK, I think you're ok with this board, that's the 1 I've got (well, mine is older) I'm almost 75 kg and I use it in any conditions, I could actually have a smaller one when it's glassy, but I like to have some volume for choppy conditions and to paddle from spot to spot.
Regarding the vent plug, everyone has a diferent opinion, in my case, and from my experience with windsurfing boards, I agree with DJ , only unscrew it when it's really hot or when you go in the plane.
IMO, If you unscrew it every time after a session, it damages the join and the internal thread and you'll end up have water filtration through there.
Also, when I do unscrew it, I tape the plug on the shaft of the paddle, so I don't loose it, it doesn't get sandy, and I remember to put it back in as soon as I grab my paddle (because the worst thing you can do is going in the water without the plug, that kills the board)
I'm looking forward for a hot day to try WM's bubble trick !!