Not going to read the whole of the thread.
Personally - we never buy any food or drinks at the cinema.
We take 500ml bottles of Coke in - nice and cheap at your local supermarket.
If we're desperate for popcorn, Tesco sell a lovely "Cinema Popcorn" - not warm obviously, but tasty.
Pick & Mix - a lot cheaper at the supermarket.
There is of course no legallity regarding taking your own food in and in my own opinion, as long as you're not taking the **** (fast-food etc) then you won't be stopped.
Remember the cinema can refuse you entry to the screens if they do see you bringing your own food, although they cannot forceible check you.
However if they asked to look in a bag and you said no, then of course once again you could be refused entry - and without any kind of refund on your ticket.
So keep it sensible and you'll have no problems - but no moaning if you are refused entry, remember they don't have to let anyone into their screens.