Welcome to the fly trap.... sorry, forum.
If you are already a retro devotee you won't need warning that there is no turning back once you embark upon this path........
I have just built that which you describe, based on an Orange P7 frame.
Unfortunately it belongs to my daughter, so it's not for sale, but it is possible within your budget.
As for builders, there are more than a few RB members in your neck of the woods, so I'm sure with a bit of bartering a deal could be done.
Have a look in both the mtb classifieds for suitable frames and bits, and also slap a couple of wanted ads up too, plus Gumtree (in Exeter, Plymouth etc) and ebay are decent sources.
"Try not to punch any clowns"