there are a few sites dedicated to this, the Natural Death Centre has a list of member sites, but they're all in East Sussex, Ecocoffins have a search engine but it's broken, maybe they can help over the phone? Woodland Burials only list a member site in Hastings.
Hopefully your funeral director should be able to help with this?
I have 2 friends that have been burried on private land. You just have to check with the local environment agency that you won't contaminate any water. And the green form from the registrar has to be filled in by the land owner and sent back to registrar. It is a really lovely option. One family made a coffin the other bought a willow coffin on-line (you can get anything on-line now can't you!!!) http://www.naturalendings.co.uk/buy-coffin-now.asp