Its a quirk of statute.. In most situations things are legal unless made illegal. For things that are illegal we often have reasonable excuse arguments (e.g. criminal damage, self defence etc). In this case the offences against the person act makes it illegal to have one or to provide drugs or tools to enable it. Sec 58, says she must not cause a miscarriage and you must not help her to achieve the same - you risk life in prison. Sec 59 makes it illegal to supply tools or drugs for the same.
An Act to consolidate and amend the Statute Law of England and Ireland relating to Offences against the Person.
www.legislation.gov.uk
The Abortion Act, basically over rules this. Had it been done today, they would have probably amended the offences against the person act.
But it doesn't really matter, Abortion is widely available and the legal test very low. Personally I think it should be recorded, even the provision of do it yourself drugs. It would be very easy for a woman to fake few pregnancy tests and phone up for abortion drugs in under 10 weeks and then pass them to someone who had say been refused an unlawful late abortion. There seem to be a dozen or so cases like this being investigated.