This is something that actually happens with regular occurence. Ultimately they cannot stop you taking pictures in public space and they cannot make you delete them. They can look at your pictures but they can't confiscate or make you delete them (if you delete them they are easily recoverable i believe ayway!). Other cases are saying tripods are causing an obstructure etc.

Half the time, the police don't even seem to know the law themselves regarding photography.
+ they always appeal for photographs when some trouble occurs.

The terrorism act was bought in to give the police stop-and-search powers as a deterrent against terrorists carrying rucksacks full of bombs. It is blatantly misused now at sensitve sites.