Wish I knew. My front doors have remote locking but I never could find something that would work for the rear door. You might check some of the UK forums like www.lrenthusiast.com. I bet they have an answer.
You can buy and install an aftermarket kit to lock and unlock the door. And if you wire it in with one of the front doors you would be good to go for the remote. I think they have them at Parts express for around $20.