Brexit row over vaccine is identical to blue passports.

Could the UK legally have approved vaccine earlier / changed passport colour as an EU member? Yes.

Have other EU members done so in practice? No.

Would the UK have done so in practice, sans Brexit? Almost certainly not.

EU membership, in practice, is about more than adhering to European laws. That is why Leavers dislike EU and Remainers like it - membership is not just about legal practicalities but realpolitik, and also principles of sovereignty vs collective action.

Would it have been possible to rethink UK membership of EU so that in practice Britain acted more autonomously, taking full advantage of all legal flexibility? In theory, yes - this is what Cameron wanted. But it would have been difficult and controversial with other members.