As far as UK law is concerned, as long as there is no intent to defraud anyone, any person is free to use whatever name they like without the need for any legal documentation. (For example, I can legally call myself Fred Bloggs on Mondays, while choosing to use the name Hiram J Gobelheimer III on Tuesdays. On Wednesdays, I can be Henry Smith during even numbered hours and Julia Sexbomb for the other hours. It's all perfectly legal).
However, other countries take a different view and only recognize a single name. Similarly, UK 'officialdom' (e.g. HMRC, bank managers, etc) prefers that each person is only known by a single 'official' name. (You can't have two passports in different names unless you have dual citizenship of two countries).
UK law permits a married woman to continue using her maiden name as her 'official' name, if she chooses to do so. It would seem simplest if your partner was to continue using her maiden name for all purposes except 'social' ones. i.e. she should leave her passport, bank accounts, etc unchanged but she should let it be known that, when away from work, she wishes to be known as Mrs ManiacPony.
If your partner wants to be able to have bank accounts, etc, in her new 'married' name, she may find it necessary to open additional bank accounts. (e.g. she could keep her existing bank account, with Bank A, as Dr NiceLady, while opening a new account, with Bank B, as Mrs ManiacPony).
Chris