If you park on your driveway & can run power cables to your van, a simple immersion heater kit, a timer and some heavy duty cables will do the job for less than £100.
If you can't run cables to your van, a tankless gas water heater that runs off butane/propane bottles (bbq gas) will set you back about £200 - £250 with fittings,.. this option requires carrying a gas bottle in your van so you need to inform your insurance.