well, technically as long as it is aiming for an hard brexit it can withdraw unilateral whenever it wants. The two year period is a safety blanket to negotiate a better deal while still enjoying the existing rules.
According to the text of Article 50, a withdrawing state remains bound by the Treaties (and thus by EU law) until either (1) an exit agreement is made and enters into force; or (2) two years pass after Article 50 is invoked.
So the UK can't 'withdraw unilateral whenever it wants' without creating a giant legal problem as its erstwhile partners attempt to enforce its (still extant) obligations.
Sure, if the UK is willing to violate its treaty obligations, it can do whatever it wants. But the context of the discussion was the legal situation under Article 50.
So the UK can't 'withdraw unilateral whenever it wants' without creating a giant legal problem as its erstwhile partners attempt to enforce its (still extant) obligations.