In terms of actual, proper established crossovers or shared universes of note, I can take:
Pat Mills' stories:
Savage
Flesh
Invasion!
ABC Warriors
Nemesis the Warlock
Ro-Busters
(Even though it's a bit of a stretch to imagine Savage and Nemesis as sharing a universe but there you go...)
Rogue Trooper:
Rogue Trooper (classic and Friday flavours)
Venus Bluegenes
Mercy Heights
Tor Cyan
The '86ers
Ian Edginton's stories:
Ampney Crucis Investigates...
Stone Island (?)
The Red seas
Stickleback
Leviathan
Strontium Dog and direct spin-offs
Sinister Dexter and direct spin-offs
Judge Dredd and direct spin-offs (Anderson, Death, Chopper etc)
What about all the Invasion/Savage stuff though, with Britain being invaded by the Volgs at the end of the twentieth century? Is all that still part of the Dreddverse's past history?
Again, depends who you ask. Wagner notably didn't do anything to explicitly contradict this in when he wrote
Origins, but he also chose not to reference it at all.
I'm not particularly enamoured of the idea that Strontium Dog's world is the future of the Dreddverse.
Same here, though weirdly I don't mind the idea of Dredd and Alpha meeting (if they could come up with a better way of writing it). Al Ewing wrote a Dredd story called
The Americans about this very thing a few years ago, that at least attempted to tie up the loose ends of the Dredd/Stront paradox.