The peer-reviewed working party report by the Royal College of Obstetrics and Gynaecologists is here. The findings of the committee hinge on the following evidence:
1) That the foetus is not conscious by that time, being kept in a state of "a continuous sleep-like unconsciousness or sedation".
2) That the neural connections allowing the experience of pain (by relay of neural signals to the cortex) are not sufficiently developed.
Recently, UK parliamentarians (including the Prime Minister, David Cameron) have begun making noises about possible downwards review of the 24 week limit. This follows bills introduced by various US states to limit late term abortions on the basis of the putative ability of foetuses to feel pain, most notably that of Nebraska which has decreased the upper limit to 20 weeks. Another 29 states have bills scheduled for the 2010-2011 legislative seasons. Of course, none of those bills will now pass if the lawmakers are interested in scientific evidence (and, of course, the separation between church and state means that the lawmakers will most certainly not be considering any religious views!).