Thanks to the good old Freedom Pass I use buses a lot around here, an area with a very diverse population and masses of foreign students thrown in. And I must say that the people most frequently plonking themselves in disabled seats, when there are plenty elsewhere, are very often young or non Brit, and frequently both.
I understand that disability is not always obvious, but when you see people,moving fast enough on and off the bus, you do have to wonder.
More than once, when someone evidently in real need of those seats has got on, and the 'plonkers' are showing no signs of moving, I have suggested that they do. Nobody has yet refused, but I would obviously prefer not to have to say it.
I have to assume that at least some of the non Brits don't understand the signs, or like to pretend they don't, though you'd think they'd be obvious enough.