There is a lot of confusion over this issue, largely cause by the likes of the Christian Legal Centre and others, who seek to twist and distort things to suit themselves. (Honesty is what they say it is!)
The point of the law is that, when a woman has carried and given birth to a baby, other than in the most extreme cases, where it can be demonstrated to the courts, that the woman is clearly unfit to be a mother, she must have the automatic right to keep and care for that baby. However it was concieved.
This is a recognition of the most basic bioligical reality of a mother. A woman who has given birth is generally, emotionally and intellectually, a very different person that when she first concieved. These are biological imperitives, part of basic humanity. The law cannot outlaw, basic humanity.
English law, in particular, has a number of common law provisions, dating back to medivel times, recognising that a woman who has given birth is a very special case, legally.
If the woman chooses to give the baby to another couple, it is an adoption. But she must never be forced to give up a baby she has carried and given birth to, because of a commercial contract.
The notion of expenses was termed to allow a woman, who decided she must keep the baby, to do so. If the payements were defined as a fee, then a commercial contract would exist. And there can be no commercial contract for events that are unforseen.
The limitation of reasonable was included to reduce the possibility of women demanding increasingly huge sums to desperate couples.
But I am very curious about who brought this case and the motivation behind it.