Perhaps they shouldn't have emblazoned the bus with the "suggestion" then? Never mind that the £350 million figure isn't even correct. What's so sad is that it didn't even need to be the figure they stuck with -- anything that sounds like "so-many-hundreds of millions" would be equally large in listeners' heads, so they could have stuck with the ~£170 million figure with no effects to the central message -- other than being truthful this time.
However much Brexit supporters bang on about the perceived dishonesty of "Project Fear" (and some of those, eg the emergency budget proposal of Osborne's, have turned out to be exactly that), it would be nice if they could hold the lead campaign of their own position to account in the same way. Whether "let's give £350 million more to the NHS" is technically a promise or not, it was incredibly misleading, and should be denounced as such.