Hi tommy welcome to TONMO.com
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.
where in the UK are you?