No, it isn't. The Surrogacy Arrangements Act 1985 explains that negotiating or pretending to do so the conditions of a surrogacy arrangements is forbidden, and anyone who attempts to do so will be considered guilty of an offence. However, it is not illegal to pay your surrogate to reasonable expenses, including travel costs, maternity clothes, etc.
Read the full article on: How Does Surrogacy Work in the UK? – Law, Cost & FAQs ( 48).