I think the only criterion for "recovered from covid19" is "not dead from covid19".
But even that can't be measured with any accuracy because no one fucking KNOWS!
Even in Australia they started limiting the tests quite early on in March - by then they'd done something like 40 000 tests and only had tens of positives. DS1 had a cough/cold/something before schools closed, I kept him off school of course but couldn't get him into the doc because he didn't fit the criteria imposed for testing - he didn't have a temp above 38 degrees, he didn't have a dry cough, there was no confirmed exposure to either a covid positive case OR a recent returnee from overseas, he hadn't been overseas himself either - so they wouldn't test him.
My friends' parents were on the Ruby Princess - which was badly mishandled - all passengers let off without any restriction BUT the day after they'd got home, they were informed by text that 3 positive cases had been on the boat. My friend and her kids live with her parents. My friend also works in an aged care facility, with vulnerably patients. She had to call in to tell them, they told her to stay off work for 14 days and get tested. My friend was the one pushing for her parents to get tested, NOT the NSW govt/health dept, DESPITE the known contact etc. - and when they were tested, one of them came up positive. Totally asymptomatic as well. So they all had to isolate for 14 days, then get retested, then wait ANOTHER 14 days after the negative test, just to be on the safe side.
That's a known recoverer (my friend's parent). But they just didn't have enough tests to test everyone, even though we were testing FAR more people than the UK, so our stats aren't much more accurate as there will be many people who may have had it but won't know!
Unless they test Every Single Person with the antibody test, they're not going to know the actual stats.