As I indicated before I have two 4K monitors that are both very good.
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BenQ PD2705U (27" 4K) looks to be £399
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AOC U32P2CA (32" 4K) looks to be £420 but the 27" is £320
The BenQ has better color accuracy but is more expensive as I use it for photography and design.
The AOC is really nice and there is a 27" as well that is way cheaper and it works smoothly with mac.
I'm in the USA got them from Amazon and it looks like you can get them in GB.
btw, there are
multiple threads on monitors for the mini so you can pretty much get all the references that you could ever need.