Selfridges now have an unpasteurized milk dispenser in the food hall. You ask for a bottle, they give you one, you put your money in the machine and it fills the bottle. As the machine is owned by the farmer, you're buying the milk directly from him, hence how they can sell you unpasteurized.
Having heard so much about it, how the taste differed etc, we bought some. £2 worth, it's £3.50 a litre. Had a sip, tasted like normal milk to me. However I am pretty lactose intolerant, and so I haven't really drunk milk since I was a child. But if anyone wants to try unpasteurized milk, now you know where.