Spud,
His point was that we would take back control of that amount, so potentially it could be spent on the NHS.
But that "point" was a lie. First, a significant portion of that amount comes back in the form of various grants and incentives, not least to the economically deprived regions that need it most.
Second though, imagine that it cost you £20 to go to car boot sale every Sunday and one day you decided not to bother any more, thereby saving yourself the £20. Good plan right?
Er, probably not if you made a £200 profit when you did go. That's the point - the economic benefit of paying the net amount we actually pay (not least from access to the single market) dwarfs the payment itself. Funnily enough though Boris didn't feel the need to put that on the side of the bus.