Not so far, the only thing you are bound by at present is "Code of Practice for the Safety of Loads on Vehicles", (3rd Edition) Available from the Stationery Office (previously HMSO ISBN0115525475 Priced at £10.50), which contains details of current legislation and advice.
If Crash Testing does become the means to measure Van Mounted WFP machines by, then Insurance Companies will have some sort of requirement in the future, some are certainly aware of Crash Testing at the moment.