Thought I'd post an update!
So I spoke with my immediate manager a couple of days before speaking with HR and he took it quite well, in fact both did. Very professional. No "silly" questions, very respectful, honest (ie. "I don't know much about transgender people", which is fine) and looks like things will be fairly "by the book", which suits me. At least that way as long as I know the rules were all on an even playing ground. They also will have to do a little bit of homework methinks which is kinda nice to be the catalyst for community education

The funny thing is that my managers' wife is also a Rachel

So yea, slightly awkward when he asked me what name I will be changing to. He then at a later date (confidentially) asked me how I thought my colleagues would react to my "new name". I stopped, thought for a moment and replied, "Well, I've been Rachel since before I started here or met anyone at work, and no, I won't be changing my name." I think that kinda put him back a little.
I also gave a little spiel about how I think vast majority of people will be respectful and that I am choosing to transition here because I believe they will all take it well. The first part I mostly believe, the second...

maybe a little bit of poetic license. I'd transition there even if I knew it would cause waves.

So it's all good! For now. I'll try keep you posted for updates as they go!