Seems that the UK market is not big enough. In China Apple had no problems with ceding control of their local servers to the Chinese government [1]. Not to mention of using forced labor (mostly Uyghurs, against which China is committing acts of genocide [2]). So, Apple and human rights my ass!
None of those investigations showed that parts supplied to Apple came from Xinjiang. Only that one of their suppliers operates a plant there. These suppliers are huge conglomerates with plants all over China. Apple is one of a tiny handful of companies that actually audit their supply chain, but they only audit the chain that actually supplies them.
From now on though any kind of audit in China is going to be almost impossible. They've started arresting researchers who perform due diligence reports for revealing 'state secrets'.
I don't think it has anything to do with the market size, which after the Brexit fiasco and the pandemic has shrunk considerably, but it has the most to do with the UK labelling itself as a democracy while behaving like a dictatorship when it comes to surveillance and other "think of the children" laws.
1. https://www.nytimes.com/2021/05/17/technology/apple-china-ce...
2. https://en.wikipedia.org/wiki/Uyghur_genocide