A bit disappointing to see you're being down-voted, based on a glance at that wp article, it links to a lot of relevant research (and the gp's statement can hardly be seen as controversial to anyone that's paid even cursory attention to social (in)equality and social studies in the past decades?).

It'd be interesting to see a breakdown of the UK on the various countries though.

That data is a proxy for income mobility. It's measuring the correlation between generations. As I mentioned in another reply, it's not measuring upward mobility, but mobility in general. If a country had a future generation do worse economically it would show up as having better mobility.