So I'm planning to start HRT shortly and I'm currently part-time at home, and out in public where practical. However many of my colleagues live near me, so it's difficult to be out more without being out at work. I work in the UK with good legal protection and for a large blue chip with excellent LGBT policies. My question is when should I inform HR of my status? I'm thinking of telling them soon, just to start a dialogue and agree a way forward with them, does this sound like a good idea?

To answer a question with a question, how soon before you plan on going full time? If you are thinking a year or more, it's early to tell them. If you are 2 or 3 months, it's time to have a talk. You want enough time so they can prepare for you but not so much they will be asking you if you are ready yet.

There are various degrees of being 'Out at work'. Plus just how sure are you of being recognized presenting as female as Mr. X?

I worked for the largest employer in a rural area where I was living. I was seen daily by some 500 people, all living in the area. At 6ft tall in a world filled with shorter women I was nervous about present as female. Often in male mode I'd come across or see people from work. Hard not to when there are only 2 supermarkets  and a Walmart, and the place I worked at employed some 1700 heads. The dropped jaws of my support group members when they first ever saw me presenting as male spoke volumes. I never had issues the other way around that I know of.

You can also out yourself to selective others at work you either run into, or know you will and they will know. Hopefully they are friendlies and keep a secret. If you are truly recognizable in female mode you may also just be another of the faceless people we all see while we are out and about. Or, just a fleeting that woman looks like _____.

No need for HR involvement until it is full-time time when all the official paperwork needs changing and the gender diversity classes to get scheduled for your work mates.

Thanks for your thoughts. Although I'm getting itchy to do it, in reality it will be year until I can go full time. I have bumped into work colleagues coming out of my front door, they know where I live. I'm happy to handle this as a situation if it arises though.