I like to think that they would argue something along one of two lines
(1) If a complaint was made that they breached CAP Code clause 7.1 (Truthfulness - No marketing communication should mislead, or be likely to mislead, by inaccuracy, ambiguity, exaggeration, omission or otherwise.) "Our claim can not be dis-proven and as so many people believe it, it is de facto true".
(2) As a catch all "It is our opinion and we have a right to express that opinion as protected by The Human Rights Act 1998".
Bullshite of course. They wouldn't stand up in a court of law but for an advert?
Anyway - it doesn't seem to matter as apparently the
Atheist Bus Campaign raised around £150,000 in its first nine days and for comparison the
Alpha International posters campaign raised
only a few thousand They may have the occasional rich backer but in general it seems their followers aren’t too eager to dip into their purses.
There are now
Atheist Bus Campaign adverts in the underground. They take the form of quotes the text in multiple light colours.
Isn‘t it enough to see that a garden is
Beautiful without having to believe that there
Are faries at the bottom of it too? - Douglas Adams
I‘m an atheist and that‘s it. I believe there‘s
Nothing we can know except that we should
Be kind to each other and do what we can
for other people - Katharine Hepburn
It was, of course, a lie what you read about my religious
convictions, a lie which is being systematically repeated. I do not
Believe in a personal God and I have never denied this but have
Expressed it clearly. If something is in me which can be called
religious then it is the unbounded admiration for the structure of
the world so far as our science can reveal it - Albert Einstein
That it will never come again
Is what makes life so sweet. - Emily Dickinson