You'll probably find that, contractually, most UK airlines forbit accepting tips and/or gifts. Occaisionally, I've had pax buy chocolates from the duty free for the crew, which I think is fair enough to accept as it would cause offence not to accept it and it's shared among the crew. Anything more than that and you can be putting your job at risk, especially if the pax has been upgraded, or received some other kind of special treatment.