You could use a 16A industrial outlet, but this must be protected
at no more than 20A, IIRC. However, it won't meet your criteria of
fitting a standard electrical box.

It is providing the 16A protection that is the issue. I'm quite happy wiring
the flex into a 20A junction box rather than a plug/socket arrangement.

What is the lowest profile 16A fuse or MCB enclosure that is available? I
have plenty of wall space, but it can't jut out more than a couple of
centimetres, so I can't use a consumer unit. In fact, MCBs are so thick, I
doubt any enclosure could be made to fit. Presumably it isn't possible to
get BS1362 16A fuses? I don't want to run yet another circuit back to a
dedicated 16A circuit, especially as I have enough power available through
the current circuit.

Christian.