I think Reddit might be fundamentally flawed because of its voting system and its need for high moderation to make it workable. Mods influence so much power over what people are and aren't allowed to talk about. You use the example of CasualUK which is indeed a very good if slightly "police state" sub but it only exists because the other UK subs became unviable.

The old defence has always been "if you don't like it go away and make your own sub", unfortunately with so many obvious subs being squatted its difficult to get exposure to newer communities.

From memory we have:

/r/unitedkingdom - Mostly teenage lefties, often resembles a 6th form common room, mostly dislike the UK.

/r/ukpolitics - Similar to above, slightly quicker on the banhammer.

/r/england - cybersquatted and shutdown for years by a power mod, just opened back up but the power mod won't give top rank over to active mods.

/r/scotland - A borderline anti-English hate sub, harsh group think in most threads.

/r/baduk - The less said the better.

I don't know what the solution is because as i said, reddit is flawed at a base level. The only positive change i can think of is to cap active participants in a sub to a specific small number.