They haven't pierced the corporate veil as far as I can tell - it's not moot or any other executives that have been personally fined. Also as far as I'm aware the UK doesn't have debtors jail.

On the contrary. Online Safety Act breaches can carry personal liability and up to 2yrs jail.

moot doesn't own or run 4chan anymore. The current owner and admin is Japanese, residing in Paris at this time.

https://en.wikipedia.org/wiki/Hiroyuki_Nishimura