I've just done mine (in the UK) - our Deed Poll system allows us to change any or all of our names... but also our
titles. So I changed the whole lot: first & second name, and I went back to my maiden surname. But I also changed my title to 'Mr'.

UK law allows us to call ourselves by any name or title we choose as long as we're not trying to pass ourselves off as having qualifications or honourary titles that we don't have. So you can call yourself Mr, Mrs, Miss, Ms., Mx. regardless of your gender or sex.... but not Dr, Prof, Lord, Lady, etc. unless you can prove you have a right to use those titles. And of course, you can't choose an offensive name.
Such a shame. I would've loved to have added 'Lord' as my title. Or perhaps 'Prince' - yes, I like the sound of that. 'Prince von FTMDiaries' would've been fun.