If most surrogacies, as you say, work - then what exactly is the problem that needs to be solved by changing a woman's legal relationship with the baby she is carrying?
There's potential for a baby to be in limbo, at the moment, and for unscrupulous parties on either side to exploit the other if they feel like it. Intended parents feel anxious that their genetic child won't be given to them. Surrogates feel possibly more anxious that something will change in the IPs lives and the baby will no longer be wanted, or that the baby will have a disability and this may change the situation (it doesn't, but these are the fears people have). There are also situations when a IPs break up before the PO comes through, which is complicated, or when the surrogate's marriage ends and her partner is no longer willing to sign anything or be part of anything. It's unclear what the law actually is and what it's supposed to be doing. So the law is not, if you like, functioning well as things stand.
Surrogacy in the UK is not a terribly rare occurrence and even if it were, it involves a child's life and should be provided for in legislation.