I am extremely prochoice. I believe that it is remarkable what medical intervention can do to keep babies alive and can be a wonderful thing, but this baby wasnt born with a life, it was saved. I do not think there is any point in lowering the limit as with the 20 weeks scan sometimes not happening for women til they are 22 weeks this would be too soon. However the 24 week rule is not final, if there was serious risk to the mother or the child a termination of pregnancy can be performed at ANY stage of the pregnancy. In my county you would find it virtually impossible to get an abortion for social reasons past 15 weeks (so how you feel Nox is virtually in place anyway) after 15 weeks the cervix needs stretching and it becomes more complicated hence doctors not liking to do it, and in most NHS cases this would be the same across the country. There is a such a small amout of terminations performed after even 13 weeks, people and doctors do not abuse this time limit.
As said above the only person I know who has had a termination past 20 weeks without having medical problems with the preganacy was a 15 year old girl, whose mother had just died, the doctors felt it would be of serious risk to her mental health and allowed for her to have a termination.