I suspect the Americans have been keeping what's left of high-end UK steel frame-building in business for years. Italian frame-building too, probably.
Road-bike fashions in the UK are surely a result of continental peloton-watching, and when was the last time a Jackson, Rourke or Mercian was seen in a peloton even in the UK, much less on the continent? If they are still seen anywhere in competitive cycling, it's probably in UK time-trials.
Apologies for banging on about the '70s, but my bike-spotting started in the mid-70s, around North London area, and I remember that back then you would scarcely, if ever, see an Italian-framed bike on the roads. No doubt a few people had something stashed away that they had imported themselves, but I don't remember ever seeing them. If we are talking about top-end bikes, you were just as unlikely to see a French-framed bike, maybe an occasional Peugeot... I remember one of my local bike shops had on display in the window a white Peugeot which must have been near the top of the range, perhaps a little dated... 531db, sprints/tubs etc., this in a shop which probably had nothing else in stock anywhere near the same quality. Back then I never anywhere saw any Italian bike displayed thus, and if they'd had it, you can bet they would have displayed it.
In my neck of the woods, mid to late '70s, if you ever saw someone out on a really nice bike, if it wasn't a Raleigh/Carlton, it was usually a Holdsworth/Roy Thame, Shorter, Ken Ryall, Roberts, Bird, or Dave Russell, probably because that was the stuff that any of the local time-trial fast-men who got their photo in Cycling Weekly could be seen riding. I can't remember exactly when, but some time in the '80s, this changed, and any fancy bike you saw on the roads, especially in London, usually turned out to be a Colnago, Rossin, Gios, Ciocc or Olmo. I wasn't looking at Cycling Weekly much any more so I don't know if the same thing had happened in there.
I doubt that there was, on average, any qualitative difference between the UK-built frames and the Italian ones that replaced them. I guess the UK builders favoured Reynolds tubing by default and the Italians favoured Columbus. So yes, the fashion changed, as it always does, and of course those who stood to profit from the change could stoop to some pretty low tactics... I remember a big advert in the back of Cycling Weekly, somewhere in which it was suggested (in small print) that you get rid of your "classic English wheel-wobbler" (that is what it said) in favour of something new, probably Italian, at least in name, and, by implication, something miles better. Also, regardless of frame quality, it seems those selling the Italian stuff can get away with using a lot of advertising which works on an emotional level, and provokes glamourous, often unrealistic associations... If you are given a choice between Milan-San Remo and London to Holyhead, which are you likely to choose?... What about if you are Milanese?
"Oh, Italian crap", responded a guy on an ancient Holdsworth
I suppose whoever said that, he was jesting, especially since even as early as the '30s, Holdsworth had a model named; 'Stelvio', and what's more, many of the UK framebuilders, Holdsworth included, spent the second half of the '60s boasting about the 'Italian' features on their frames; sloping or semi-sloping fork crowns, long-point lugs, often Prugnat, (which are French anyway..)