Skipness One Echo
It depends.
In the example you use ie a bottle, I believe there is no statute on the subject, although it may fall under regulations the DfT can make for items carried on aircraft.
However, the rules used by the airports authorities are that if you wont fly without it, then you can't use the airport, and therefore you can't fly.In that case you get landed (subject to imigration controls).
As to getting it back, No idea. When it is taken from you, you have to sign a form in errect surrendering posession. Before the current security restrictions, this form used to outline if the item could be returned to you, and how you go about it.
As for other items, they may fall under the aviation security act, eg a flick knife (yes, people do try to fly with them) and no, you wont get it back, as it's an offence to have one in the UK, which also applies to many other items which people try to take in hand baggage.
Smudger
No, it's not theft. If for no other reason than there is no intention to dishonestly appriopriate.