"If you want to ride in the middle of the road pay road tax!"

My best come back to this is one so far - this woman pulled out into a cycle lane, blocking it, to pull out, having clearly seen me coming.

After I told her off she shouted 'You don't even pay road tax!' so I shouted back 'No, and neither do you!'.

I'd say something like that, or 'did you know that road tax doesn't exist?' You then usually get a second or two to start on the explanation.