Well, for me the supercar class isn't so exclusive. A supercar to me is one which simply excells in track performance over defined road practicality. I mean there are tons (also literally lol) of cars out there not particularly geared for the road, but are by the manufacturers considered road cars. For example, any Lamborghini (except for the Gallardo) would be beyond road practicality and be more track based cars, thus making them supercars. Or how about the Enzo? Which I do believe is street legal in the United States, however it isn't road prone, but geared for track. Or another example would be the Ford GT. It's completely street legal, but again, not exactly something you'd take to and from work. There is simply just too much power in the punch for them to be considered just road/sports cars.
IMO, a sportscar is usually more tamed, meaning it has more road practicality than track performance. For a long time the Corvette remained king of this hill. It was a sportscar, it had adequate or more luxuries, and the gas mileage was awesome. The ride wasn't as smooth and the road noise was deafening, but just imagine how an Enzo would sound with its engine blarring away behind you. Imagine how untamed an Enzo would be in traffic from one traffic light to another. Mustangs are more road geared, but because of this, their performance on the track suffers - making them sportscars.
Thats basically the basis of my judging what is and isn't a sportscar or supercar. If the Caterham is even considered a car in whatever country it is produced, than I would gladly call it a supercar. IMO, a supercar is all about track peformance, but that doesn't necessarily mean it needs 600hp-800hp.