The Surrogacy Arrangements Act 1985 is the law that regulates surrogacy arrangements in the United Kingdom. Under UK law, the woman who gives birth is considered the legal mother, and her husband the legal father. However, it is allowed to transfer parenthood by applying for a parental order or adopting the child.
Also, it should be noted that surrogacy arrangements are not enforced by UK law, even if you've signed a contract. In other words, even though surrogacy is permitted in the UK, there is no legal certainty that you will end up being the legal parents of the child.