When I came back from my first taste of continental cycling, I felt very much the same. Riding long climbs giving ample time to feel the rhythm, reaching summits and plateaux where the air is crisp; this was what cycling should be about. There aren't climbs of the same dimensions in these islands, though some are big enough to give something of the flavour. All too often they are in areas where the roads are sparse and options are necessarily limited.
In my experience, no part of the UK comes closer to recalling the essence of continental upland cycling than the valleys of South Wales. It's a post-industrial landscape so roads are denser than would otherwise be expected. There are endless options for riding valleys, hopping between them, and riding the ridges as well. An important caveat is that when I rode there in the 80s and 90s I didn't regard any road as off-limits; a bit more caution in route selection might be required nowadays, but as compensation there may be better off-road options than I remember.
Purely for illustrative purposes, here is a route I created just now. All roads I've ridden and enjoyed at some time:
Not quite the heights you'd've found in Tuscany, but it does have the brontosaurus profile you're looking for.
Some climbs from the area have been mentioned such as the Rhigos and the Bwlch. The latter is a three-headed monster, and it would be a nice challenge to complete a ride including all three ascents. Beyond me nowadays, sadly.
I would say that the climbing is generally easier than in the Pennines, but there are plenty of tough options if you want to seek them out.
You can easily link to the adjacent Brecon Beacons which has some of Britain's best climbs. Perhaps they can be usefully thought of as a single region. You could spend many days riding around and feel you'd hardly scratched the surface.
Doing the Lairig Ghru would give you 2,275 feet of climbing but "you may need to push your bike for some segments of this route".
