My 10p's worth: I've just had a look at reg 553.1. It appears that the BS1363 13A socket is cited as a preferred outlet. Providing certain requirements are met, such as shutters, and prevention of contact with live parts while inserting the plug, the use of other outlets is not precluded where it is necessary to distinguish the function of the circuit, or prevent danger. In this example it would seem reasonable to have a circuit distinguished for the use of European appliances. Danger could arise with the use of adaptors, none of which carry BS certification, and it would alleviate the very significant risk of danger should a visitor try to force the 2 live pins of European plug into a UK socket. It is possible to get a connection by doing so. Providing the level of protection afforded for the particular type of socket is the same as it's native country, and that level also meets UK requirements, I can't see why it would not be permitted and beneficial to do this. We commonly see non-standard BS1363 sockets in UK homes for lighting, freezers, outdoor supplies etc. The 2A, 5A, 15A, IEC, figure of 8 etc are not BS1363 sockets either.