The clear unequivocal position in law is that it is not illegal to take photographs in the situation that you have described - illegal meaning commission of a criminal offence, or even for that matter, stretching it even to apply to civil liability.
However, given that you are on their premises, they are able to stop you from taking photos and if you continue from that point forward, they do have the right to ask you to leave, failing which you could be guilty of criminal trespass. Note however that they do not have any remedies for photographs already taken nor the right to demand that you surrender or delete any photographs.
The sticky situation then arises if you are halfway into or just starting the meal - do they have the right to ask you to leave. My view (which is not as unequivocal as the above two paragraphs) is that if this condition was not drawn to your attention prior to acceptance of the contract, they would be in breach of contract by not fulfilling their end of the agreement; ie to allow you to consume your food in peace. This is of course, merely for academic argument and it is not likely this will become an issue in real life as most pple will guai guai stop taking photos and then consume their food.