I witnessed that too on Ryanair some time back. A fellow pax was warned that if he refused to delete his picture the police would be called on landing as it was 'illegal'. I put this down to a grumpy FA but maybe there is more to this?
Certainly not illegal, I doubt the Police in the UK would bother turning up at all.
"He took a photo of a friend on a commercial flight? Dear God we'll be right there!"