Bestwishes, abortion is only legal at a 'late' stage (past 24 weeks) in the UK for medical reasons. Of the 0.1% that occur at that point, the vast majority are not viable.
It is very rare for a viable pregnancy to be terminated past 24 weeks, because the are only allowed if the mothers life is at risk, and not many women want to terminate at that point as they have become attached to the baby. So yes, in these very very rare circumstances, the baby's heart would be stopped in vitro and they would not be born breathing.
Also, at 24 weeks onwards as it is an abortion out of necessity, it would be treated the same way as a stillbirth or miscarriage. The mother would be taken to a bereavement suite, allowed time with the baby and photos, footprints etc, and given the option of a cremation.