It was the Abortion Act of 1967 (see last part below) that provided such wide exceptions for wholesale slaughter. From: http://members.aol.com/abtrbng/abortl.htm
Historically, in Anglo-American law, abortion had been criminalized, at least from the point of "quickening" (c.15-18 weeks) and often severely punished. Liberalization of abortion laws in both countries began to occur in the later 1960's.
Henry Bracton, (1216-1272) "the Father of Common Law," apparently regarded abortion (at least after 5 or six weeks) as homicide and it seems that at early Common Law abortion was a felony, and, therefore, a hanging offense. Later commentators, Coke and Blackstone, held expressly that abortion after quickening was not the crime of murder, but a separate crime (a "grave misprision"). It is unclear whether pre-quickening abortion was still criminalized. The Miscarriage of Woman Act of 1803 ("Lord Ellenborough's Act," 43 Geo. 3, c. 58.), introduced a statutory abortion scheme in England. Pre-quickening abortion was made a felony and post-quickening abortion was a capital crime. In 1837, with abolition of the death penalty, 7 Will. 4 & 1 Vict., c. 85. § 6, the quickening distinction was removed and all abortion was punished as a single felony. In 1861, the Offenses Against the Person Act, 24 & 25 Vict., c. 100, § 59, introduced a replacement statutory scheme, where, as before, all abortions were felonies. In 1929, the Infant Life (Preservation) Act, 19 & 20 Geo. 5, c. 34, was passed. It supplemented the OAPA and included a defense for bona fide efforts to save the mother's life. A common law health exception to the OAPA was introduced in 1938 by Rex v. Bourne, [1939] 1 K. B. 687, 3 All E. R. 615 (1938). Finally, the Abortion Act of 1967, while maintaining the general prohibition of abortion, introduced broad exceptions for genetic defects, and the mental and physical heath of the mother. Under this law, abortion is generally permitted if a pregnancy is unwanted, as childbirth is seen as more of a health threat than early abortion. However, this law does not apply uniformly throughout the U.K., e.g. Man and Jersey.