"What do we think that a young adult should know after they have completed 13 years of compulsory education here in the UK?"

Well, as they have followed different paths through school, have different talents and abilities (or disabilities) and all progress at different rates, there's no answer to that, is there? I could say a basic level of numeracy and literacy, but one student's good is another student's bog-standard is another student's impossible. I could say they should have a well-rounded, intelligent and sociable outlook on life, but, grand though that sounds, it's just waffle. I could say sympathy and tolerance for all other humans, and a high level of environmental consciousness, but ditto. I could say that the generations above them have the responsibility to pass on all the best of what they themselves have learned, so that the young 'uns can stand on the shoulders of giants, but gosh, that's mired in all sorts of murk, innit...