To get the best value - ebay and the best, most exhaustive, most honest description you can muster. With lots of photos. Its the bad news that sells old cars on ebay.
Quickests and easiest way is probably Gumtree - i've seen cars sell in minutes there. But be prepared to repost the ad regularly as a bit like the classifieds here you fall off the front page pretty quickly.
Ebay (as an auction) is probably the most convenient though - you only have to deal in person with whoever wins the auction, rather than gumtree / classifieds where you might get several tyre kickers before you make a sale
Read up on the advice (from somewhere like Which or Parkers.co.uk) about the practicalites of the actual sale itself - make sure you know what you are doing with the registration documents, read advice about how you recieve and handle payments etc, just to make sure you're not getting speeding tickets and parking fines turning up months down the line