IIRC, you can fit a two round pin plug into a three round pin socket, with occasional judicious use of a dry plastic pen top in the earth (top) hole to lift the lock. Those used to wantonly disregarding safety provisions on UK sockets will be very familiar with the procedure.

Obviously, this is an idiotic idea and should not be attempted by anyone. And unnecessary on the vast majority of sockets - I can only think of a couple of guesthouses where I needed to consider, and then thoroughly dismiss the very notion of, doing this.