You can get a version of the standard 13A plug + socket with a slightly smaller, i.e. more square, earth pin. The plug will fit any 13A socket, but a normal 13A plug won't fit in the special socket.
I'm not sure where to get them though (our scenery workshop has them to stop you plugging in your own power tools).
One thing to think about is a lot of computer accessories use wall warts. If you really want to prevent plugging of other stuff into the computer supply you will have to consider how you will deal with these for example by using foreign imports or by screwing/locking them in an enclosure of some kind so the BS1363 sockets you use for them can't be used for anything else.
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