Christians have hit back at the atheist bus adverts with adverts of their own.

They have the right to advertise like that and so do the atheists

well,
it is not allowed to make false claims in advertising in the UK (hence the "probably" in the original ads). So if this is not a photoshop christian wet dream then it is illegal

it is not allowed to make false claims in advertising in the UK

Does that apply to politicians' campaign promises, too?

Don't think so. I haven't researched this, but as far as I know political propaganda is not covered by the same laws. Someone else may know different though.