Then I suggest you either don't book tickets or don't attempt photography if you do so, as you are generally agreeing to allow someone to do just that in the Ts & Cs when you buy the ticket. And you might find it difficult to argue in court that someone doing something that you've allowed them to do is an assault.
I don't think this is correct. You can't just put any old stuff into T&Cs. If I put "all customers are to be strip searched before the performance" that doesn't actually give me the legal right to do that, even if a customer bought a ticket, thus implicitly agreeing to T&Cs.
I agree that on the whole taking photos in a theatre is rude and shouldn't be tolerated, but the venue has no legal right to insist you delete them, nor any right to confiscate the camera. The most they can do is ask that you stop, or eject you from the venue.
Copyright is also a red herring, as unless you are trying to sell the photos on or otherwise profit from them (without the copyright holder's permission) you are not in breach. Think about it. If just taking a photo of something was a breach of that copyright we would never be able to take any photos with a brand name or logo visible.