The legitimacy (legal authority, laws) of UK gov to deliver the A50 notice --or-- is not disputed. Parliament granted gov this authority. (And have been since squabbling with amendments to its "Withdrawal Bill" who expeditiously to curtail gov's authority to negotiate with EU gov.)

The legitimacy of UK gov (excluding parliament) unilaterally to interpret provisions of LISBON in order to "revoke" its own A50 notice is a legal question (argued in this thread with political/legislative speculations) that is not the subject of SCS appeal for review by the ECJ.

More UK "cherry picking" --this time straight to the throat of separation of powers, republics hold so dear.

BWAH!

Diversity is the key to economic and political evolution.