I use Insurance Choice (brokers) who are OK, not so impressed with customer service but the price was OK for a self converted campervan. (self installed windows/gas/240v etc etc etc – lots of potential for fire/explosion!) Think the actually policy is with LV.
Close second was Campton Insurance Brokers, will give them serious consideration come renewall time as despite several calls, IC have still failed to send me an updated policy doc showing increased mileage I paid for.
Others are Adrian Flux and Frank Pickles.
You can get a 10 – 15% discount on the above policys if you become a member of the SBMCC for £15. http://www.sbmcc.co.uk Also discounts at lots of other suppliers if you decide to improve your van.