I would say clifford would be the best....
if its gotta be thatcham catargory approved, then its gotta be a concept clifford 300, 500 or a 600, has the cossie got nice alloys? Coz the only difference betwene the concept 500 or 600 (apart from the price) is a module that detects the car being tilted or jacked up (i.e jacking up to nick your alloys) & sets the alarm off! if he doesnt have expensive alloys thens there no point in paying the extra for that module, Ive got the 500 with all closure & remote start & when I try jacking the car up or washing it with a jet washer the proximity chirp still goes off! so i dont see what the point is in the extra module IMHO. seeing as the proximity sensor goes off anyways, & if the proximity sensor carries on going off then the actual alarm will go off to.
But I think that remote start cancels out the thatcham catagory 1!! so you might wanna ask ur ice shop about that if he/she wants that.
I have two leds on my motor (or had now! - sold car) one on the dash in the centre & one in the centre console & I had no problems with my ICE being stolen & that was a softop.
hope that helps