Most insurance companies will insure a wheelchair-accessible vehicle if you give them the vehicle registration number.
Other options:
Fish Insurance. A company that specialises in insurance for adapted vehicles.
They can be contacted on 0333 331 3770 www.fishinsurance.co.uk
BP Insurance. A company that offers specialist mobility insurance.
They can be contacted on 0800 970 4054 www.bpib.co.uk