They don't have to provide a reason but there still would be an underlying reason why they've rejected you, their reasoning would be that your ID was dubious, a quick little correspondance with either the citizencard people or with the Home Office (which backs the Citizencard project) and suddenly they'd change their tune on the cards. While it's not going to get you in on that night and while yes they will have refused you service off their own backs and they're entitled to do so, they won't be refusing anymore people once they've had someone read them the law on the issue.
The whole point of the citizencard system is the government provided ID you mention, it's just been hived off to an NGO, the PASS hologram is legal verification, thats the point of it, if you wanted to take a bar to court over it, you'd win because it's legally binding ID. I'm not saying you should or anyone would but if a bar's right of refusal extends to citizencards, then it also extends to passports and drivers licenses as their legality as forms of ID in the eyes of the government is exactly the same as citizencards. And if it extends to all those forms of ID, then it's essentially saying bars can turn away perfectly legal people with verifiable ID who are showing no signs of violence, no signs of drunkenness and aren't known to be troublemakers because they don't like the look of them, or simply because they don't want them in their bar, which is not in the spirit of the law that provides bars with the right of refusal.