EU State aid rules will continue to apply until the end of the transition period. However, for up to four years from the end of the transition period, the Commission will be able to bring a State aid case against the UK on facts arising before the end of the transition period.
Ben should also note that the state aid rules will be transposed into U.K. law via the European Union (Withdrawal) Act 2018. That’s the same act which ends our membership of the EU.
So in law the only path to achieve Bens desire is to pass the Withdrawal Agreement which include provision to end the state aid rules.
Alternatively, legislation to change the law (post Brexit when it’s been transposed into U.K. law) could be implemented.
But that means parliament changing its mind which some think to be undemocratic.