[onion2k]

If you make a deal to buy a bus (or 1000) today, what tariff would you be incurring as a result of Brexit?

None. There would be no tariff at the moment. Yay!

But... if you buy 1000 buses you don't pay for them right now unless you're a crazy person. You pay for them in the future, and there might be a tariff then.

And if we're post Brexit, what tariff is the UK going to impose on a UK manufacturer that kills it's business?

If the UK leaves without a deal then the WTO tariffs are applied instead of the zero-rated EU trade zone agreements that are in place right now. For 'light commercial vehicles' that tariff varies from 10-22%. In other words, a UK bus manufacturer will never sell a bus in the EU ever again as long as the WTO rules are in place.

The good thing is that the withdrawal agreement or No Deal brexit is only the first step on the long and arduous journey that is Brexit. We would work to negotiate a new trade deal either with the EU or with each individual member. So, hopefully, that tariff wouldn't apply for ever. Eventually we'd be able to sell buses in the EU again, assuming the manufacturers manage to survive that long. It's probably that they'd move their businesses to the EU before that though, and employ EU citizens instead and pay tax to the new country the business resides in. It's not good for the UK.