I've used Conrad Anderson but you have to go to their premises in Birmingham for fitting - it will cost you around 400-500 fitted (price it up on their website) but its well worth it, especially if you have back problems like I do.
Not too sure about use on UK roads; I got it for frequent long distance journeys to SW France.
But its an excellent option and I'm just about to pull the unit out of its second home ready for them to install into a third Terrano since I bought it about 9 years ago.
By the way DIY isn't an option - although you can remove most of it easily enough, fitting involves clutch and brake switches, a special cable to the throttle pedal (as its Fly By Wire normally) and fairly detailed setting up and tuning.