What steps need to be taken by a prospective bridegroom, before the wedding takes place, so that he can adopt his new wife’s surname, as opposed to the other way round?
None. You can use whatever name you choose, and marriage is probably the easiest way of formalising a change of name. A marriage certificate is generally accepted by banks, the DWP and the like as proof that you have indeed changed your name. However since the bridegroom in this case is behaving slightly unconventionally it’s possible he will run into difficulty, in which case he could have a change of name deed prepared by a solicitor for a relatively small sum.