On any tax documents I fill out, I have to declare all past names used (which means my birth name, my current name, AND the married last name I had for a little bit). Not sure how it is in Britain, but that's how it is here.
I understand the desire to have the option to go stealth. But, as others have pointed out, the preoccupation with passing 100% of the time can be a dangerous rabbit hole that has the potential to keep us from living an amazing life. Before I started transitioning, I was very worried about what a long and arduous process it may end up being. I had no idea what to expect, and my imagination definitely tried to fill in the gaps. I understand the fear.
But, thankfully, my actual experiences transitioning have quieted that fear. I hope you find the same when/if you begin your own transition.
I am in the awkward, middle part of my transition. There's no hiding it. I am pre-top surgery (and very large chested so the male presentation is obviously incongruous with the female-appearing body) and only 5 months on T. I am out to all my friends and coworkers now, and everyone is beginning to change pronouns. There's no hiding my transness at this stage in my transition. My fear was that people would turn their backs to me... that I would lose people. In reality? My current friendships deepened, and are more supportive, honest, and real (from both sides) than I ever dreamed possible. My coworkers are understanding and learning about trans issues so they can continue to support me.
Long story short: I have never felt more love - both from others and for myself - than I do being out, even during this awkward-and-obvious-tweensition year. People will surprise you. And you will surprise yourself with your own strength.