Hi there ...
I can't help you for UK Laws, but just to let you know:
The Default value that my Insurance reserves for Car-Hifi is about 1100 Euro, and i was able to pump this up to about 3300 Euro.
In my Insurance contract this is called "additional value", and is ONLY for fixed things -> attached to/built in the car, no loose things like discman or similar ...

So here in Germany it IS possible, at least if you are a customer of the "Provinzial" Insurance ...


Nils