UK road legal harnesses will be E-marked and have a normal seatbelt style press release buckle. The background to this, as Dogmaul says, is that you should be able to release it one handed. The turnbuckle style that you find on the full house harness sets are not road legal because when you pull the lever they do not pop themselves open, they simply release the catch so you need to actually pull the straps out manually. I have had both in my Clio, but found the illegal (but FIA approved, ironically) ones that I have currently a lot more comfortable to wear in bucket seats because they dont have to run vertically and horizontally in order to get the one handed E-approved buckle across your lap. E-marked road legal harnesses also have a lot less in the way of adjustment available. At the end of the day its up to you; if youve decided on harnesses in a road car with proper buckets, Id recommend you get 3 or 4 point and use the existing seatbelt anchoring mounts, dont get clip-ons mounts cos they rattle like hell, dont go hardcore and get 3" wide straps cos they are plain uncomfortable and pretty heavy, 2" is fine, and go for aircraft style turbuckle for a better fit. You could always just put your standard belts in at MOT time, or find a tame MOT garage who dont bother too much about silly details.