The £10k fine was potentially over the top for throwing a party, but that was the whole point of those fines. To act as a major deterrent to people having parties. At the time people were dying in large numbers and the NHS was massively struggling. The consensus amongst younger people appeared to be that they'd be okay and didn't need to worry about Covid, ignoring that they'd then potentially infect other more vulnerable people. It's along the same lines as the death penalty is meant to deter people from committing the worst type of crimes.
Not saying I agree with this approach but trying to explain what I believe the legal thinking behind it is.
Still doesn't change the fact that the government are awful and should do much, much better with the first step being resignations.