In the US, personal belongings stolen from a vehicle are usually not covered by auto insurance but instead by a person's homeowners or renters insurance. In the UK, it appears that comprehensive motor insurance polices allow for claims due to loss or damage to personal effects in a car, up to a stated limit, after an excess fee is paid.

If you have a comprehensive motor insurance but the excess fee is too high, you might discuss reducing the fee with your insurance company. This likely will make the cost of your insurance premium rise but allow you to have an excess (deductible) that you could pay more easily than your current one.

If you unable to reduce the excess fee, you might look into a personal belongings or contents policy. Contents policy typically seems to be for household items but it might be possible to find one that covers autos.

Personal belongs or personal possessions polices typically extend to cover accidental damage to or loss of items which you take out of your home, so again you will need to check and see if they allow this coverage to be taken out on a motor vehicle.