thanks for the info, now that the beer's worn off I'm afraid my guesses have not improved. It still has more "permanent" features that point to a British frame, such as the BB threading and the standard quill. But that size seatpost doesn't help things. Are you sure it's really 26.2 and not 27.2 and that it's a proper fit? The next thing to measure (with digital calipers) are the outer diameters of all 3 main tubes: if they measure exact even mm, they are metric...fractional mm mean Imperial, AKA British dimensions.

The only trivia I can add is what I've read: so many frames with Nervex Pro lugs came with a different brand of BB shell because adding that shell increased the cost dramatically. The fact this has all the Nervex Pro bits, including fork crown, makes me think it was a top-of-line frame (in its day).