I think you've somehow not understood what i said/what we're talking about here. They make sockets designed to be attached to the outside of your house, the actual socket is in a little weather proof box that you open, plug in whatever, then close it back over the plug to keep it safe from water etc... but the socet within that box is pretty much identical to one you would find inside a British home and you're using the same Brittish three pronged plugs into it with all the added safety benefits they give.

The problem with those, when it comes to christmas lights, is that christmas lights often have the transformer and/or the switch to control the pattern of the lights built into their plug, making the plug a lot bigger than the standard UK plug and the aformentioned external sockets are built to only accept a standard size plug, there's no extra room for a bigger than normal plug.

There would be nothing unsafe about plugging the lights into it, if only you could fit the cover down over it, they're just not made that way as the typical use for them is gardening tools etc... and to make the boxes bigger around the socket would cost them more money and reduce profits only to benefit a very small part of their potential market.