Is this coincidence or part of a bigger trend? Why are they selling/shutting down craft beer breweries? Does Brexit play a role in this?
It's complicated - there's always company-specific reasons as well as big picture stuff. Obviously, making it harder to export food and drink doesn't make the UK more attractive to invest.
But the UK market is a brutal place to be if you're a multinational trying to move large amounts of beer. Particularly during lockdown, there's been a big move of "craft" into supermarkets, where margins are generally razor-thin, and it's hard for relatively new entrants like Kirin to force their way past the rows of Brewdog etc.
Also routes to market in draught have been closed off with the likes of Greene King and Heineken buying two of the biggest independent pub estates in Spirit and Punch, which has effectively tied them to their new parents. (qv the comments in this thread about beer availability)
Then going to the specifics - Magic Rock made a lot of good beer, but they never really had that "core", must-stock brand for the masses in the way that Brewdog has with Punk and Beavertown has with Neck Oil and Gamma Ray. Cannonball is the closest they've got, and may have seemed to outsiders like something that could have been developed, but at 7.4% it's never going to be mainstream in the UK, that's just not our culture. The 3.9% Saucery could have been that beer but just hasn't really taken off. Also, I don't think their branding really works in smallpack on supermarket shelves.
Whereas they did rebrand FourPure and are widely regarded to have made a complete mess of it, I thought it was not terrible, but not great either. They seem to have been pushing it more as a supermarket brand - but see above, that's a brutal place to be. I've enjoyed what FourPure I've had, but again they never really seemed to establish a must-have beer, although I wasn't really paying attention.
So I think Kirin have decided that they've given it a go but the UK market is just too difficult, and they'd rather concentrate on some of the "cool" brands they've got elsewhere - it's perhaps no coincidence that this announcement comes just after they bought Bell's in the US. But I always thought it was a bit ambitious in the first place, just because they didn't really have enough footprint in the UK to really break through.
London Fields is very different - it was bankrupt, and Carlsberg bought it for pennies with no illusion about it being "cool". But it was a very cheap way to get a brand with "London" in the name, which is something that could work in less sophisticated export markets where just buying a beer with "London" in the name seems kinda cool. And Carlsberg didn't really have a "own-label" "craft" brand that could be bundled with their other brands into a package to put on a bar. But then Carlsberg UK "merged with" (took over) Marston and no longer needed to develop a second-line craft brand from scratch, as Marston already had some.
But that was fine, as the London Fields project was a cheap bet that hadn't cost them much, presumably the name will disappear and an independent will buy up the kit.