Certainly no law in any of the UK jurisdictions I can think of. It could well be a breach of Ryan Air's COC's though. Unfortunately, in UK I could very imagine the Police intervening though! There is an ongoing problem with Police (and our equivalent of the 'pretend' Police known as PCSO's) thinking you are not allowed to photograph or video them (or public buildings) and this has caused MANY problems with photographers, journalists etc.