Of course there is an element of realpolitik here. China will be the dominant power in the world in the latter half of the 21st century. As such, it is (to an extent) given a free pass which other countries that commit similar acts do not receive. Likewise, Saudi Arabia has been indulged for many years.
It just goes against the grain to welcome good relations with them. And I can’t help feeling that the British Left would be a lot more vocal about a regime it perceived as “right wing” which behaved in a similar manner.
https://en.wikipedia.org/wiki/Persecution_of_Uyghurs_in_China