28 stores is small by UK supermarket standards. Sainsbury’s alone have over 1400. I can’t reasonably consider Booths reflective of trends across the country, perhaps for the reasons you suggest.
OP’s question as to whether the UK is rejecting self checkout on any level isn’t really addressed by the example in this article.
Though the BBC is obviously identified most with UK, it in fact has many international publications. This article focuses on the US, with only a reference to “Booths in the UK”, a very small supermarket group I have never heard of before.
Self checkout in the UK is commonplace and largely popular, though some of the general customer criticisms in the article are familiar to me as a regular user of them.
I use Feeder too, it’s entirely satisfactory.