Airlines investigated over claims they deliberately split up groups then charge for a

[DaveReidUK]

I think you may find that the CAA have a fair amount of clout in this case, regardless of what the law may or may not say.

"Airline seating practices are clearly causing some confusion for consumers. Airlines are within their rights to charge for allocated seats, but if they do so it must be done in a fair, transparent way.

Our research shows that some consumers are paying to sit together when, in fact, they might not need to."