There do seem to be a few transmen in prison.
However despite, as I understand it, a Prison Service Directive stating that people should be housed according to their lived gender (i.e. if you are a 'genuine' transperson), I'm not aware of any transmen being housed in the male estate.
This is Claire Taylor www.portsmouth.co.uk/news/addict-locked-up-for-vicious-attack-on-90-year-old-1-1252232, a heroin addict under an indefinite sentence for public protection, and by all accounts & appearances a very dangerous person.
Claire is transmasculine
www.facebook.com/smashipp/posts/taylor-claire-taylor-is-an-ipp-prisoner-that-smash-ipp-have-been-supporting-they/958430890976895/
bristolabc.wordpress.com/2018/07/16/action-alert/
However there doesn't seem to be any interest in moving Claire to a male prison.
'Ben' is (or was) a transmale young offender in HMPYOI Foston Hall
www.mkcollegeolass.co.uk/news/blog-article/hmpyoi-foston-hall-learner-wins-mk-colleges-student-of-the-year-award
Carlos Delacruz is fairly convincing transman, albeit possessing a vagina, and it is been reported to have been segregated www.thescottishsun.co.uk/news/scottish-news/3181313/carlos-delacruz-falkirk-fake-willy-prison-male/ while prison staff worked out whether to place Carlos in the male or female estate.
So, so far I am not aware of any transmen in male prisons.
And for obvious reasons.