I would say that it was a violation of the Fifth Amendment and tell you to contact the ACLU, but alas, you do not live in America. English law is a complete mystery to me.

If there is any law in England similar to the Fifth Amendment ("You have the right not to incriminate yourself."), then I would go with that defence, say they came - unannounced and without warrant - onto private property, and verbally coerced you into admitting guilt. Not a perfect defence, entirely, but the part about private property should - I think - be the main focus of your defence.