In their risk modelling it doesn't matter whether you caused the accident, they still load (to some degree) your premium for non-fault accidents because something about your behaviour is risky - eg you park in sh!tty Asdsa car parks or choose on street parking spots etc etc.
500 quid for a repair might sound bad, but quite probably over 5 years you will pay this and more for letting them know.
Of course, it is your legal responsibility under the terms of your insurance to inform them of any incident. So please don't imagine I am advising you to not follow these contractual terms ;-)