None of the branches of the UK governmental system care about international law, so this is not an impediment.

The judicial system is constitutionally required to ignore it. The legislature is mostly controlled by the executive. And the executive only respects international agreements when they are convenient.

The requirement they are referring to is

> must pass all the laws needed to comply with that agreement ahead of time

Which is already met, so it wouldn't change anything.

If they ever decide to change the laws, they will have to renegotiate the agreement, which is a change.