May I remind you, the UK does not have a constitution.
The UK doesn’t have a single document constitution but it does have a body of constitutional law.
It most certainly does have a constitution. And in that constitution Parliament (not the Government) is sovereign. Or, as it’s usually expressed, ‘Nothing may be done which Parliament cannot undo’. The Brexit Referendum was only binding because Parliament (not ‘the Government’) chose to make it so. The British Constitution is one of representative democracy, not direct democracy. Which is why referendums, a device of direct democracy, rarely happen and are usually messy.
This interim agreement over the transition period cannot be finally ratified unless and until Parliament approves it, and that won’t happen until the full Brexit settlement (large parts of which still have to be negotiated) is put to it. ‘Nothing is agreed until everything is agreed’.