Well, it depends how much publicity you want of your horse.
The popular American advertisement websites (such as craigslist) aren't as popular in the UK.
A very popular website to sell horses on is:
www.horsemart.co.uk
It's the 2nd link to appear if you put in "horses for sale" in Google, although it costs £15. If you put a horse up for sale on a site like this (or like horsetrader, horse and hound) you're going to look more reputable than a free site.
On the other hand, this site:
www.stabletalk.co.uk
Is a very popular site for those in the UK looking to advertise for free, and you can advertise anything equine related. It has thousands of hits a month and the ads are regularly updated.
Hope i helped.