Agreed with the first part. The percentage of folks who stay on, after studying at the UK's prestigious HE institutes do not constitute the demographic "advantage" they talk of. These are mainly low skilled workers from the eastern end of the EU and other states.
Do you also have evidence for the major PISA tapering by the Chinese? Apart from using Shanghai as the Chinese representative.
As for the tests are useless, according to whom? Looking at the ranking trend with each nation, and their corresponding HDI scores, economic growth figures and other measures of prosperity there are strong correlations between success at PISA and a successful country.