The recently-passed European Union (Withdrawal) (No. 2) Act 2019 (a.k.a. the Benn Act or the Surrender Act, depending on one's politics) seems to be unprecedented in that it creates a legal obligation for a single Crown subject—the Prime Minister. Specifically, § 1(4) states that:
The Prime Minister must seek to obtain from the European Council an extension of the period under Article 50(3) of the Treaty on European Union ending at 11.00pm on 31 October 2019 by sending to the President of the European Council a letter in the form set out in the Schedule to this Act requesting an extension of that period to 11.00pm on 31 January 2020 in order to debate and pass a Bill to implement the agreement between the United Kingdom and the European Union under Article 50(2) of the Treaty on European Union, including provisions reflecting the outcome of inter-party talks as announced by the Prime Minister on 21 May 2019, and in particular the need for the United Kingdom to secure changes to the political declaration to reflect the outcome of those inter-party talks.
However, usually legislation includes details of the punishment to be administered to those to break the law. For example, § 1 of the Offences Against the Person Act 1861 states that ‘Whosoever shall be convicted of Murder shall suffer Death as a Felon.’
This Act doesn't mention any punishment, however. What, then, happens to the Prime Minister if he refuses to seek an extension (which seems to be Johnson's plan) and thereby breaks the law? Is there some sort of ‘default’ punishment for lawbreaking in England & Wales that is used when no more-specific punishment is stated?