(2-part answer):
You state that you're not sure what you can take through the security checks.
You can take any foods you like through those checks as long as they're not in 'liquid' form (and in quantities exceeding 100ml). I've used quotation marks there because the definition of 'liquid' is quite wide. It includes creams, gels, pastes, etc. So you can't, for example, take a large jar of jam through (not that you're likely to want to!).
So you can take your own sandwiches, biscuits, muffins, etc without any problems.
http://www.direct.gov...avelintheUK/DG_078179
If you're buying anything to take onto the plane (whether that be food, cosmetics, gifts or whatever), remember that some budget airlines are very strict about only allowing one bag to be taken onto the plane. Anything you buy should be placed into your hand baggage, not carried in a separate bag.