You don't say what kind of wood was used. Some woods, such as redwood (here in the U.S.), cypress and the old standby, cedar are nearly self-preserving due to the various natural oils they contain.
But even those types of wood weather to a shade of gray over time. I'd suggest that you pressure wash the fence to remove dirt and grime and after assessing the structural condition (is it sagging or leaning; are the fence posts rotting in the area where they go into the ground?) and you decide it's worth salvaging, I'd use a product like this:
http://www.homedepot.com/p/Wolman-5-gal-F-P-Cedar-Exterior-Wood-Stain-Finish-and-Preservative-14415/204642357
It's (and others like it) are excellent preservers of wood but with a light cedar-shade tint which will also help the fence retain a pleasant wood grain color for the next several years. If possible, it would be best to apply the stain with an airless sprayer like this:
http://www.homedepot.com/p/Graco-X5-Airless-Paint-Sprayer-262800/100634354 ... not only will it make a snap of the job, the stain will penetrate much better than being brushed or rolled on. You can rent these industrial strength sprayers very reasonably here in the U.S. and, I would expect in the U.K.