Hawke House Ltd ( www.hawkehouse.co.uk
) in Gosport do 3" self draining foam for £16.62 per Sq Ft. according to their price list.
They may do other thicknesses as well.
When I did an upholstery course there they said the only downside of the self draining foam is it may not be as good as protecting you from hard landings as it compresses more, so this might be worth checking if you need the foam to absorb impact. If your set up is such that you tend to stand up in rough conditions it won't be a problem of course!