You can buy eggs, petrol, Mars bars and Strand fags. Advertising, you see. These days, if an ad promises undeliverable things, the company gets punished. You are not allowed to say that Mars bars will make you big and strong or that Esso petrol will make your car go twice as fast. If you want to see your bus slogan as an ad in that same vein, then its perpetrators should be punished. The £350 million could never have been delivered in any case, as that sum was based on a gross figure. The real net figure was around half that sum, a fact concealed by the dishonesty of the leave campaign, but, even so, it was still undeliverable as other economic consequences of brexit (as we've seen) would have prevented it. Of course you can advertise with slogans, but in the case of the bus message it was an attempt by lying slogan to sucker people into voting brexit. You may try and defend it, but, but any reasonable measure, it was indefensible right from the outset.