Are there any these days? Obviously I know there are postboxes at
airports (though I don't think they're always that easy to find), but
I haven't paid a great deal of attention to airside postboxes - and I
can imagine there may be whatever security concerns nowadays too.

I'm thinking specifically about Stansted at the moment, but would be
interested to know about the other London airports too, indeed about
airports further afield as well.