Yep, we could veto long term budgets, but annual budgets can be passed with a qualified majority.
If we were playing hardball with the EU the approach now would be:
1. Formally revoke A50
2. State publicly that we consider May’s deal to be the opening Brexit offer and need for renegotiation
3. The EU will state that the deal cannot be renogtiated, and certainly not without A50 being triggered
4. We generally cause mayhem inside the EU, vetoing budgets and such, until they agree to renegotiate the deal, ahead of us retriggering A50 without the 2 year no-deal deadline gun to our head.
It would probably all end up back at the ECJ, who would side with the rest of the EU that Britain is behaving unlawfully. Or the rest of the EU would gang up to pass legislation that screws Britain, for example in the financial sector.