It is the entire post code that counts not just the first part,which only indicates the town or general area. For example BB7 2PR could be a listed as a higher risk area than BB7 2PQ or vice versa. I have just made the last part of the code up to show the idea.
My post code is SG8 but that covers a town of 43,000 inhabitants and 8 or more villages over 2 counties, all types of property from social housing, flats to large 5 and 6 bedroom detached houses worth £2 million and more!