S - Supermarket
Supermarket/supersite - Large independent trader with a wide range of cars from several different manufacturers. Prices are usually cheaper thanks to lower overheads, while pre-registered imports can also be on offer to bargain-hunters.
Make sure cars on offer are UK specification and be aware that warranties may not be as long as from a franchised dealer. Also check that you're not losing money as a result of a poor part-exchange valuation.