Parliament has the final say in the UK, much like the Senate has the final say on treaties in the US.
And the government is the representative of parliament in negotiations.
The fault is of the UK governments that it didn’t and doesn’t get the parliament’s blessings before negotiating.
The previous 2 years was a waste and therefore, unless Boris can prove any deal he makes will pass through a parliament that loathes him, any potential deal he will propose (he hasn’t proposed one yet) will also be a waste.
Perhaps parliament should negotiate with the EU then?