Hi Izzy,
It's cos it's a group 19 and you are leaving it out on the road. Also, the L postcode doesn't help. I am L32 so I know how you feel! Try offering a higher excess but to be fair, £650 is about right for the class of car. Mine was £880 but I have business use and 20K miles per year so cant really use it as a comparison.
Regards
Ken