In Great Britain (not Northern ireland) abortion is legal up to 24 weeks if 2 doctors authorise it. Beyond 24 weeks, abortion is permitted if there is a medical reason, i.e danger to the mother's life or in the case of foetal abnormality. As with all questions answered here, we can give factual information but the enormous emotional and psychological issues surrounding abortion cannot be covered here, specialised counselling should be sought.
I would advise talking to someone at your local Family Planning Clinic, they will advise you on your options and even provide counselling if that is what you would like. Most terminations are carried out around 12 weeks.
Coemgen, flashpig states in another question of his that he is a single, straight bloke, so we can assume he is referring to someone else or this is a wind up, which I hope it isn't.
Kags is correct with 24 weeks however you should know that the further on you are in your pregnancy (even if you a well within the 24 weeks) the less likely you will be able to terminate the pregnancy for "social" reasons. 12 -16 weeks is usually the cut-off for reasons other that congential abnoramilities or medical reasons for the mother.