I should think the vast majority of twenty-somethings in London on £30k+ salaries are repaying student loans, so you'll be taking the 54% figure.
I agree this dataset is not ideal for these sort of calculations, but the figures are supported by my anecdotal experience.
I kind of wish the data presented was more thorough, but that's probably because I'm too close to the industry - the headline is certainly catchy, and the comparison over time certainly more interesting, I just wanted give a heads up about the actual % figure not being representative.