Cheapcarsonline.co.uk doesn't sell cars, it only leases them. It's basically a finance company which can set up a leasing arrangement on new cars with one of several manufacturers. At the end of the lease period, the car has to be returned to the supplier.
The part of the same company which 'sells' new cars is called ukcarsdirect.com. The cars aren't actually sold by the firm. They only act as an agent for the purchaser, to seek the best price. The contract to purchase is made with the main dealer located by the company, which then offers to provide finance on the deal.
As stated, both cheapcarsonline and ukcarsdirect are simply part of a finance company. They might be able to help you find (and finance) the car you're looking for but, in general, adding a 'middle man' to any deal is more likely to add to the price than to reduce it. You should also look closely at their terms. For example, if you back out of a purchase deal with ukcarsdirect, they'll charge you �500. (That's on top of any deposit you might lose with the main dealer who's actually supplying the car).
Chris