The dealers will have some pretty hefty margins in their used cars. They have the monopoly on the nearly new new stock, so will artificially inflate the prices to, A- make more money, B - give the impression that they hold their money well, or C - try to get you to buy a new car instead. They will also use their high used prices as a reason to not discount a new equivalent.
The deal you get will also depend on what you have to trade in. If it's a car that they can retail, they won't get their money back until they've sold it and in some cases, sold the car they take in px against that. If you have no px or a £500 banger, you'll be able to negotiate harder on either a new or nearly new car, especially if you go in armed with a quote from a broker like Drive The Deal etc.