I agree that all-you-can-eat sushi places tend to be of low quality, you wouldn't find Japanese people eating there, because they tend to be run by Koreans or Chinese (sometimes south east Asians too). This is also the case in most western countries.

It's not a buffet but I would recommend Asakusa in Mornington Crescent.

I also had a good experience in South Kensington, it was a conveyor belt buffet where they charge by the plates at the end. These are also usually not run by Japanese people, but this one was. It seems to have closed down or changed management during the pandemic though, as I can't find it on google and the ones in S Ken now have poor reviews.