There's no way reasonable doubt remains, what is the legal barrier that an appeal court is trying to deal with?
> But these would be largely symbolic acts because the government can't, at the stroke of a pen, quash a conviction
This is untrue. Judges may not like it but I don't really care. The parliament is legally soverign and the courts are subservient to interpret and apply the laws as written in the legislation. If they write a law that says these convictions mean nothing, then they mean nothing.
Common law constitutionalism is based on the fact that individual rights exist - not the right of a judiciary to have separate powers.