try www.greenplanetinsulation.co.uk
contact them to find out if the foil rolls they sell are suitable for use under your static.
Wickes sell thick and thin styrene sheets check with them the FIRE PROOF property. If only fire retardent then my advice is don't use it.
The way I would tackle it is the foil rolls if suitable stapled initially then batten around the edges with 3 mm plastic strip [no maintenance]same as used to trim around PVC doors to close gaps screw these in with bright rust proof screws. Then cross batten screw in middle into flooring but not tight so it doesn't crush the insulation allthough I don't suppose it would make much difference the small area that would be crushed.
No need to tell you but going to, it is essential you do not cover ventilation holes, as in caravans these must be kept clear.
Bob
April 2009