You have a good point. I don't know if there is advertising for religion in the UK, but there often is in both the US and in Canada. Any agency charged with assuring truth in advertising should not permit religious advertising because it is not proven, or even provable.
In Ottawa, the capital of Canada, the bus company has refused to allow the humanist "there is probably no god" campaign for bus posters. But then, they don't allow religious advertisements, either, or any advertisement which would "cause unreasonable offense"--i.e., something gratuitous. The spokesman for the Ottawa bus company was adamant that people who object to commercial advertising are not being offended on a religious, ethnic, gender or other categorical basis, and that the company is already careful to keep offenses on such bases out of their commercial advertising.
The "professional atheists" are howling about the decision in Ottawa, but personally, i'm comfortable with it--mostly because it entails a simple justice and balance.