You’re not wrong, but jail time and justice is not what motivates HMRC, or any other revenue department. They want one thing, and one thing only: The money. Putting people in jail costs money, letting rich people 'fess up and pay up is much more lucrative.
Putting people in jail is a deterrent. You do that and rich people start thinking very hard about breaking the law like that because suddenly it’s very much not worth it. They are already rich, losing a few mil is annoying but going to jail is devastating, especially when it’s so easily avoidable.
And now the rich are paying more taxes and so you make more money. Catching and convicting people of fraud isn’t exactly cheap either, and don’t forget that not everyone will be convicted.
I think that interest is added, specifically you pay https://www.irs.gov/payments/quarterly-interest-rates based on the quarters you were missing taxes.
Not sure if the numbers quoted here include that already
