HMG initially said that it would ratify the Withdrawal Agreement after it had laid it before Parliament in accordance with s. 20 of the Constitutional Reform and Governance Act 2010. This allowed limited control/scrutiny & was inappropriate considering the implications of the WA
Following pressure, including in pre-action correspondence in a case brought by @BestForBritain @TechForUK @OFOCBrexit HMG amended the Withdrawal Bill to provide for an Act of Parliament prior to ratification in s. 13(1)(d). 
If we get that far, HMG will not be able to ratify any WA until Parliament has passed implementing legislation (the Withdrawal & Implementation Bill). That legislation can be amended to provide for a referendum on whether to accept the WA or stay a member of the EU.
Time is too short though for this to happen before 29 March 2019. HMG will have to ask the EU 27 for an extension for such a vote.
