Apparently you can test positive for months after. Which begs the question... Why bother testing subsequently?
As I understand it, LFT only show +ve when you are likely to be infectious (ie, high viral load in the areas you swab). PCR is sensitive enough to pick up previous infections, which is why you previously couldn't take another PCR test within 90 days of a previous one.
Also, as I understand it, you still need a -ve LFT to stop isolating, even if your 10 days are up.
As to its seriousness, I posted before about someone I know who went to an event pre Christmas, and 75% of them subsequently tested +ve, which shows how transmissible it is, though who knows how many were infected before going. But more worryingly, 15% of all attendees are now in hospital cure to catching the virus.
1 in 7 chance of being hospitalised aren't bad odds. But its certainly going to snarl up the NHS, and I seem to recall you grumbling no so long back about the hassles getting a minor injury looked at....
Just saying