While internet sellers have lower overheads than traditional car dealers, and can therefore typically sell cars for a few hundred pounds less than an independent dealership, you should still shop around.
The largest car supermarket groups and dealers still offer competitive prices, but more importantly are open to haggling. They will also consider taking your current car in part-exchange, something internet sales companies don't do, and it's worth looking into the warranty cover you'll get online sellers typically offer only three months' cover.
There's also nothing to stop you enjoying some of the benefits of buying online while still dealing with a conventional garage. Buy a used car from a forecourt and you're not entitled to a cooling-off period, but place a deposit over the phone and the distance-selling regulations automatically come into effect giving you the same level of protection.
What Car? says
The future of buying used cars could well lie online. Some of the best internet sellers offer vast amounts of information as well as the sort of reassurance that many traditional forecourts can't match. However, don't always assume they're offering you the best price, because bricks-and-mortar motor dealers can still surprise. Always make sure you do your research thoroughly beforehand.