I wasn't aware that the entire contents of the Club Kitchen become YOUR snacks the second you set foot on board. I trust the other J passengers on your flights get a suitable refund since they presumably have no right to items in the Club Kitchen since that is YOUR stuff.

Isn't the point really really simple? The contents of the CK are there so that if a J passenger feels like a mars bar/apple/packet of crisps etc. during the flight such things are available. Until a J class passenger appropriates an item it remains BA's property, not the passenger's. A J passenger who has no intention of consuming the mars bar/apple/crisps/whatever during the flight has no business taking it because they are potentially depriving other J passengers and they are taking BA's property. That is true regardless of the fact that they may have a child at home who would love the mars bar, a fetish for feeding mars bars into airplane lavatories or mates on the flight in Y who might fancy a snack.

And just because this sort of conduct isn't sufficiently serious or clearly criminal to make it very likely that you would end up on trial at the Old Bailey or being banned by BA doesn't make it acceptable. It is selfish, a denial of the reasonable rights of fellow passengers and of the airline and symptomatic of an "I'll do whatever I want and stuff the rest of you" attitude which is sadly prevalent these days.