Good luck with your application Schism!
As Padma said, you only need to change it by deed poll. I think in terms of then changing other things like name with the bank, it can vary. I went into my bank and spoke to them there (unfortunately they didnt take me into a room, I had to practically whisper out in the open to the woman, my situation.) The woman I spoke to was great. Was very understanding and didnt bat an eyelid. I was 19 and had been on T about 2 months, so had practically no changes. I looked (and felt) about 12. She helped me out a great deal. I had with me my deed poll and that was all I needed.
Also, if youre looking into changing your passport, I think I called up and/or did it online, I cant quite remember, but all I needed was a copy of my deed poll, and a letter from my clinic. All of my documents are as male. (I just havent got the birth certificate sorted yet. But am sorting my GRC).
I hope that all the people you speak with are understanding and make things go smoothly for you. Its so crap to hear of, like Padma experienced, people being nobs!