err,just how many miles do you think you will do... then you dcide to go on hols in teh car round scotland -add 2K, then you happen to be unlucky to need regular hospital treatment for a fortnight or you have to visist a sick elderly rel. add a few hundred here and there and you'd be surprised just how easily it can creep up by 5-6K wiping out any 'savings' and posssibly even costing you more. Its very hard to accurately judge your mileage if you are an atypical user as by definition you will use your car as and when you feel like it based on whats happening in your life at that point
I'm not even going to mention the possibility of teh black box reporting you for speeding, or anything....
and you never know just when teh gov will make it law for NU to have to make their data available!