I'd be willing to bet that this particular convention has stuck around from more rigidly gender-role-dependent times not because of any particular malice towards women in modern Britain but because the country as a whole can't stand changing their quite-literally-medieval legal codes.

If that's how you'd bet, you'd lose your money.

There have been innumerable changes to UK legal codes since the medieval era, including many reforms whose purpose has been to address, bolster and roll back various forms of male-only privilege, such as primogeniture and laws that historically restricted a host of rights solely to males - such as the right to own property, the right to keep one's own wages, the right to be paid fair wages at the same rate as others doing the same exact job, the right to inherit from your spouse, the right to have a bank account and bank services, and right to be served in a pub (which was only extended to women unaccompanied by men in the UK in 1982).

In the course of the 19th and 20th centuries, numerous changes have been also been made regarding how Parliament and the monarchy operate that specifically deal with male primogeniture and male privilege.

https://en.wikipedia.org/wiki/Married_Women%27s_Property_Act_1870

https://en.wikipedia.org/wiki/Married_Women%27s_Property_Act_1882

https://en.wikipedia.org/wiki/Law_of_Property_Act_1925

https://en.wikipedia.org/wiki/Equal_Pay_Act_1970#:~:text=The%20Equal%20Pay%20Act%201970,1963%20of%20the%20United%20States.

https://www.nationalarchives.gov.uk/archives-sector/legislation/historical-and-legislative-background/

https://www.parliament.uk/about/living-heritage/transformingsociety/electionsvoting/womenvote/keydates/

nt.uk/about/living-heritage/transformingsociety/electionsvoting/womenvote/overview/thevote/

https://www.stylist.co.uk/visible-women/law-lawyer-feminism-today-history-gender-pay-gap-abortion-equality/188525#:~:text=1982%20Women%20can't%20be,Gill%20and%20journalist%20Anna%20Coote.

https://www.parliament.uk/business/lords/lords-history/lords-reform/

https://www.britannica.com/event/Reform-Bill

https://www.britannica.com/event/Parliament-Act-of-1911

https://en.wikipedia.org/wiki/House_of_Lords_Act_1999

https://en.wikipedia.org/wiki/Succession_to_the_Crown_Act_2013

When the UK's Gender Recognition Act of 2004 was in the process of being written and discussed in Parliament, there was considerable focus on the importance of not letting the new law remove or diminish male primogeniture and other forms of legally-enforced male privilege then still enshrined in UK law (keeping the Masons male-only, for example). You can read the debates in Hansard.

The GRA of 2004 as it was made into UK law explicitly says that when males receive a Gender Recognition Certificate and change their legal sex to female, they still will retain the all rights to inherit titles and property, and to serve in positions, that by UK law and longstanding custom are reserved only for males. At the same time, the GRA explicitly says that when females obtain a GRC and change their legal sex to male, they cannot and will not be considered male for the purpose of inheriting titles and estates, and for obtaining any other male-only rights and privileges that in the UK it's permissible to exclude females from, such as the right to seek membership in the Masons, to join the RC priesthood, to become a Muslim imam, and to attend Eton, Harrow or Radley.

Britain on the whole is allergic to changing oldass laws on anything

That claim is utter rubbish. If you took the time on occasion to crack a history book or two, watch some historical documentaries, or enjoy theatrical-release movies about truel-life events in the UK such as "A Man For All Seasons," "Amazing Grace," "Suffragette," "In The Name of The Father," "10 Rillington Place," "Made In Dagenham" etc, you'd get a sense of just how wrong that wholly unsubstantiated assertion of yours is.

https://www.oxford-royale.com/articles/laws-changed-britain-forever/

https://www.lawabsolute.com/recruitment-news/article/biggest-changes-in-the-law-in-the-past-decade/