Indeed, the quirk of the Westminster system is that parliament can indeed legislate whatever it likes. Parliament can pass a law that the colour orange is now legally the new black.

There is however one detail, what is passed must be internally consistent with other legislation otherwise it will fall at Judicial Review. For example if you want to pass legislation that contravenes the UK's Human Rights Act, you need to include a "notwithstanding" clause to show the courts you intended to override that law.

They could pass a bill to vacate these convictions quite easily but unlike the Buggery Act and Offences Against the Person Act 1828, they still want to be able to prosecute and protect the convictions of a subset of these people who actually did commit false accounting so the legislation is going to need to be more complex.