Your Post Code is a very important factor to your insurance company because every area in the UK has been risk assessed by insurance companies by their crime and claim statistics. Therefore your postcode can have quite a significant effect on your insurance premium; regardless of whether you are a safe driver.
Insurance companies have been able to profile crime statistics areas across the UK to see where the highest and lowest insurance risks are based and then price your car insurance accordingly.
Information regarding your post code can also explain why people who are driving the same car as you in the same town can sometimes pay different insurance costs.