To be sure there are some shocking state schools in the UK but if we are trying to compare apples with apples here, then try comparing those shocking state schools with the rural township schools in SA where there is one bathroom for 100's of kids, roofs that leak and no textbooks to speak of.
Now try to compare the better upper middle class state run schools in the UK (Note these are still free) with the upper middle class semi privates in SA, like KES or any of those (that you pay a fair amount for. And you will find the SA schools are still lacking vs their counterparts.
This is not even including the good grammar schools in the UK that are a notch above that and still free.
Then when you reach Private schooling, the level of schooling and education that can be found in the private sector in the UK far outshines anything that SA has to offer.
SO yes, there are some terrible schools in the UK and even some less desirable ones. But if you are living in the equivalent of Brackpan or Alex and expect to find a good school in your catchment area, then you will be sadly disappointed.