I fitted one, you can buy them online,I attached it to my van, I took it to a body shop and he drilled a hole and attached the socket, I then took it inside to a cut- out fuse box bought one from a Wholesale Elec shop, (it was for a shower unit), mounted a three way socket and wired it up, worked a treat.
I know what you mean, you want a hookup lead that combines a 3pin domestic socket at the caravan end, yes? I guess you have seen these on the continent, but these will of course use 2pin continental domestic socket? I've not seen a UK version of these except lashups on ebay that use a short piece of lead wired into caravan socket(do not buy one)but as pointed out, not a good ideal as it cannot use the caravan rcd protection so a better idea as pointed out is to get an external socket installed on caravan,
I wanted an awning socket but didn't want to cut a hole in my brand new caravan.
I built what is essentially a tent type EHU, it has a 16A inlet that my lead plugs into and a 16A outlet which plugs into the caravan. It also has 13A sockets with waterproof hinged lids, all protected by RCD/MCB