Don't know what the wiring regs say about it at the end of the day adding say another double socket to an existing one on a spur is no different than plugging in a four socket extension lead into a socket. I suppose if you feed your spur off a FCU then you can control the maximum current drawn by whatever you plug in by what type of fuse you put in the FCU making sure it is below the rated maximum for the cable.

I have seen it done, in our first house a 1975 build there was a spur in a studded wall that ended with a faceplate in the the two bedooms either side and it was simply spurred off a socket on the landing and it was the original wiring done by the builders.

Richard