It seems to me to be largely a matter of reasonable consumption - if you are in a hurry it seems reasonable to take the food you'd normally have eaten in the lounge. It also seems reasonable to take a few extra of an item you really like, provided you ask first to make sure you aren't depriving fellow passengers. Taking your open crisps with you is definitely reasonable because no other passenger would want to eat them, although I suppose the fact that something has no value does not mean it can't be regarded as stolen.

I would suggest that the removal of the glasses was definitely theft (assuming they weren't returned), because fittings and fixtures are not intended to be removed by passengers. Glasses do get smashed, but I doubt BA budgets for them disappearing from the inventory at the same rate as the kettle chips.

Newspapers are more tricky because whilst they are reusable they clearly have a very limited lifespan. Also as they are handed out on the plane it could be argued you've paid for one and it isn't your fault if BA's stock of newspapers doesn't happen to be where it's customers are. At this point I feel I should admit to removing newspapers from trains if I haven't finished reading them and to taking the puzzle page from the newspaper at work (not that anyone else ever takes much interest in it)

The champagne is trickier still because it is a higher value item and because the removal of an entire bottle means the removal of multiple servings rather than just one and thus potentially impacts more passengers. In this case the behaviour of the pax in question was instructive - she was acting like a thief (putting the stuff in her bag) which suggests she regarded her behaviour as theft and that it was not intended for consumption by anyone entitled to consume it.