Unfortunately, without more information on the stamp it is impossible to say. It could be worth a lot of money, or less than £1.
The best thing you can do is either buy a stamp catalogue, or view one online, such as StampWorld. You should be able to find your stamp, either on this site, a similar one or in a hard copy of a cataologue, and then you can determine it's value.
Another avenue might be to visit a stamp dealer or an antique shop and ask for advice there but make sure it is a reputable place in case the stamp does turn out to be worth a lot of money, I'd hate to think that you had been scammed.
Good luck and I hope it is worth a fortune!