The belief has to be reasonable and the fact that he had been granted a JD indicates there is a reasonable case to be made. And he has said he doesn't believe an offence will be committed and that any fines will be repaid in due course.
http://www.guardian.co.uk/uk/2012/apr/16/london-cab-firm-drivers-bus-lanes
He may be wrong but that is his belief.
No I think, for the purpose of the SCA2007, his belief must be honestly held in a subjective sense, rather than being reaosnable. This would actually be to his credit in running the kind of defence you suggest he might.
But i think he has a belief that the regulations re bus lanes are discriminatory and wrong. He might even have a belief that a High Court Judge will find the same. But he is aware and has a belief that his drivers will or might committ an offence as the law stands. Whatever he thinks of the law, whatever remedy he thinks a High Court Judge
might order (remedy under JD are at the discretion of the court) he still believes a criminal offence will be committed and a penalty provided, hence his policy of indemnity.
His best defence would be he honestly thought that JD procedings would result in a particular remedy which would be retrospecively applied so as to quash a conviction. But even then, under the SCA2007 all that needs to be shown is that, at the time the statement was made, it was capable of encouraging a crime (meanaing one existing at that time) and and he believed it would be committed...