Let's look at the full text of the advertisement. First off,
"There's probably no God"
That "probably" there, which can't help but remind one of the old line about agnostics being God-fearing atheists, is the first sign of the cowardice of this campaign. This "probably" immediately renders the whole sentence pointless. They might as well have spent thousands of pounds of donations on bus adverts proclaiming, "It might not rain tomorrow" for all the banality of that "probably". Even if the organisers protest that theirs is not a cosmic cowardice of the All-Mighty's wrath, but a liberal cowardice, a fear of offending the other subject who fully believes - and presumably is too weak to deal with such a shock to their world view, could even react violently... This, perhaps, is even worse; betraying a venal, patronising attitude that shares kinship, ultimately, with fascism.
"Now stop worrying"
When hundreds are dying every day in the middle east, in the midst of a burgeoning world recession, with unemployment sky-rocketing, the notion that all people need in order to "stop worrying" is to be disabused of their primitive religiosity, is offensively naive, not to mention politically suspect. It is also historically and sociologically inaccurate, considering the evidence that anxiety has risen proportionally as religion has declined in society. The very modern concept of anxiety, from Freud to Sartre and Camus, is an atheist one, with religion more commonly damned for its calming, perhaps dangerously pacifying, effects (Marx's "Opium of the people," etc.).
"And enjoy your life."
And finally, the clause in this message shared with every single other advertisement in the modern world, the ideological core of modern society, that persistent super-ego injunction: "Enjoy!" So, atheism makes its Faustian pact with hedonism and its politico-economic counterpart, modern liberal capitalism.
One might say that the irony of this campaign is that it makes of atheism a religion, one possessed of many of the worst features of the world's existing religions (aggressive fund-raising, self-promotion, missionary zeal, vacuous rhetoric, suspicion of, and sense of superiority over, outsiders, etc.), without any of their finer points, amounting to little more radical than an injunction to spend money, have fun, and forget about political struggle and metaphysical curiosity.