In May my new 90 XS SW will be going to "Rustmaster" who are between Hatfield and St. Albans/Harpenden in Hertfordshire. I looked into rustproofing a month or so back and came to the conclusion that they fitted the bill for me. I spoke with a chap from Stockton-on-Tees who'd done a similar investigation and concluded they suited him and was happy with the treatment.
They use Waxoyl Professional products, one for the cavity sections and another Hard Wax grade for the underside, which is only available to the trade, of which they are the sole UK authorised user. This stuff comes from Waxoyl AG, a independent Swiss company who gained the overseas distribution rights for the stuff years ago from Waxoyl in the UK who are part of Hammerite, in turn part of ICI. This stuff also has a UV component so they can see with a UV light where they have or have not coated.
I'm nothing to do with them whatsoever and have yet to experience the service myself, but concluded they seemed best for me anyway. See www.rustmaster.co.uk for details. Rustproofing discussions on the forums can be very heated as everyone has an opinion of what is or is not good practice etc, and I found the comments helpful. As you say, Land Rover ceased applying any underseal of any form in mid-December 2007. I hope this is of some assistance.