Earlier quoted context omitted.
Here in the UK we have what's known as CPSPOC, or "Criminal Prosecution Service Proceeds of Crime" unit. Their job is to recover all proceeds from criminal activity. In this case I imagine they'll try to argue that the bitcoins mined with stolen electricity are proceeds of crime, and they'll confiscate them. The money recovered will go to the treasury after the electricity company has been compensated.
That assumes they can actually get the Bitcoin. It's also likely that there won't be anything left after compensating the power company, since the mining operation was likely unprofitable if taking the cost of electricity into account.
If they can't then they can also charge the people behind the electricity theft with a bunch of additional crimes about withholding the proceeds of crime, money laundering, refusing to hand over encryption keys, etc. If the operation was unprofitable that doesn't matter either - if the CPS determine you mined, say, 1000BTC then that's what you 'owe'. If you can't pay it then they'll seize your other assets up to that amount. That money is then used to pay the electricity company, government, etc.
The law is very much rigged against you if you're caught.