So, back in January, Amazon decided to "shake up" the MGM leadership (namely they replaced it entirely).
That came to fruition in May - and the very end of the month, right when pre-prod preparations for casting etc. were underway in the UK, Amazon announced they're cancelling the project.
I'm 100% sure it's because new leadership came in swinging dick, and found the project to be too attached to the old leadership (it got approval under them). And as usual, new management cleans house, gets rid of all the old stuff that wasn't making money yet, to replace it with their vision.
So yeah first of June was a major hit for fans.