It is legal to be a surrogate in the UK. The only aspects of surrogacy which are illegal are advertising (you cannot advertise that you are willing to be a surrogate) and commercial brokering (only non-profit organisations can match surrogates with intended parents in the UK).
It is not illegal to receive payments for surrogacy in the UK (this is a common misconception) although if you are paid more than reasonable expenses this may complicate the court application your intended parents need to make. Any surrogacy agreement you enter into with your intended parents is also not legally binding, so you each trust each other to honour what you have agreed. Find out more about surrogacy arrangements under UK law.
UK legal parenthood
You will be the legal mother of any child you give birth to, even if you are not the biological mother. If you are married or in a civil partnership, your husband/wife/civil partner will be the child's other parent. It is not possible for your spouse to opt out of being a parent, unless it can be proved that he or she did not consent to the conception. If you are not married, one of your intended parents can be the other parent immediately from birth and can be named on the birth certificate with you. This will usually be the biological father, but other options may be available if you conceive at a UK fertility clinic.
Your intended parents can apply for a parental order after the birth. Once the order is made by the family court (which typically takes 6-9 months), the birth certificate is re-registered in their names. At this point, your legal status as a parent is fully and permanently extinguished. The application is mainly managed by the intended parents, who will have to satisfy the court that they meet all the criteria. You will be asked to sign some documents and will usually be visited at home by a parental order reporter appointed by the court to confirm your consent.
If you work with a single intended parent, the law is more complicated because single applicants cannot apply for parental orders. This may complicate things for you, because you might remain the child's legal mother. However, the law looks set to change later in 2017 to enable single biological parents to apply for parental orders.
Find out more about parenthood and parental orders.
Other post-birth issues
As the birth mother you will be entitled to normal maternity leave and pay, even though you will not be caring for the child after the birth. Your intended parents may need you to support them in making decisions about their child's care before the parental order is granted, as they will assume care of their child immediately from birth. Find out more about rights and responsibilities in the 'limbo' period.
You should take steps to protect your own family if anything happens to you as a result of the surrogacy. It is common for intended parents to pay for life insurance coverage for their surrogate. It is also sensible for you and your partner to update your wills. Find out more about wills and life insurance.