I think a lot more than 30 a year happen. I would count the first as a free birthed because a woman can decide at any time where an who to allow or not allow at her birth. She made the decision on the spot and during labour to go without an attendant.
I think a lot of women who choose not to have a mw present call them late and the birth go down as BBA.
Though home births are available on the NHS in practice in many areas it is not for lots of reasons. Some women who can't afford an IM will therefore free birth rather than go to hospital especially if their experiences with hospitals was traumatic in a prior birthing experience.