hmm, I hate to say that I can't help you but I know who can, pm colin, or maybe he'll stop by here soon enough since he's a moderator of this forum, anyway's I'm sure he know's where you can find one in the u.k.
Oh I forgot of a LHS in West Susex where they sell octos and stuff.
Why not try searching for it here, just type something like "octo in u.k." and I'm sure you'll find something there.
easiest way is to phone up a few LFSs until you find a shop that gets their marine fish from TMC (Tropical Marine Centre) and they can order one for you. Make sure you pick it up the same day and in its original bag so you can do the acclimatising.
Also, there is no guarantee that you wil ever get the species you want in the UK, even if they do claim they can.