Again, important to distinguish types of amendments.
An amendment that required a change to the current WA provisions on length of transition (31/12/2020 in default but subject to agreement to further years) to a new default date would preclude ratification unless the EU immediately agreed.
But an amendment on Benn Act lines that required the Government to use its rights under the WA to request an extension (or to seek Parliament’s consent before not doing so) would not prevent ratification. And such a provision could be repealed by a new Parliament next year.
