Which means you can use a European plug in a UK socket, if you first put something in the earth slot (a key works well).

European travellers are generally impressed when I show them this trick, and sometimes rightly concerned that I'm sticking metal objects into a power outlet. If there's any chance the outlet is wired incorrectly, don't do this unless you can isolate the outlet or the key.

Yeah I wouldn't rely on things being wired correctly. When I moved into my current UK house the neutral and earth were physically connected (someone had driven a nail through a cable and "got lucky"). Took a while to find the nail and fix that.

Please don't demonstrate this to anyone. It's very difficult to do without damaging the socket.

(Source: frequent socket replacements at a British university.)