There has apparently been a little bounce in the last few years, a sort of long term consequence of changes made to education policy by New Labour and the consequent movement of teachers from selective education into comprehensive education, but for example here is an article from twelve years ago:

https://www.theguardian.com/education/2013/oct/08/england-yo...

We were, for decades, a country in considerable literacy decline, and the only one of its kind. Some of this might have been a consequence of scrapping the grammar school system without giving the comprehensive system access to the same resources.

I tend to think that when Americans observe that British politicians and actors speak with a larger vocabulary or more eloquently, the people they are observing are products of the tail end of the grammar school system and the growth of a bias towards private education that emerged in its wake; the school system that turned out men and women of letters used to be available to all and now is available only to those with money.