What book would you swear your Oath on?
The European Convention on Human Rights
A traditional religious text (Koran, Bible, etc.)
A less Traditional religious text (Gospel of the Flying Spaghetti Monster, Principia Discordia, etc.)
Something Else which I shall detail in the comments
I'm not sure what the rules are for courts, these days. I suspect they're a bit more stringent than parliament. I know most courts let you choose a religious text if you are going to swear by almighty God to tell the truth, the whole truth and nothing but the truth, so help you God, but I don't know if you get a book to hold if you're affirming, or what they do with people from polytheistic faiths... Google gives me an article about the situation in Norn Iron and a .pdf of what happens in courts martial, but nothing concrete on English law other than lots of people saying it needs reforming...