Comment 12 by Tony d

@Comment 8 by Shrinking_Dogma

The is no crime of causing offence. There is a crime of causing harassment alarm or distress in certain circumstances, or behaving in certain ways within earshot of someone likely to be caused harassment alarm or distress. It is well established that this means genuine harassment alarm or distress - not merely feeling offended or uncomfortable.

That sounds fine and great but what happens if some religious type becomes alarmed and distressed by my disbelief in their religion.

If you were harassing them with your comments (e.g. by berating them in the confines of a train), then I think they could have every reason to be alarmed and distressed and you could be breaking the law. The nature of their beliefs and yours are not really relevant. As mentioned by others, it's all about the context. There's a time and a place where you can say exactly what you like, and there are other times and places where people have a right to be left in peace.