Hi peacebone, I normally don't comment, and can go months without reading on here. But as you are asking about Nottingham I thought I should raise my head again

I've been completely through Nottingham, Post Op, gender recognition certificate and earlier this year discharged. The clinic is as far as my experience been a good place. But I was very active in arguing with my specialist about some of their requirements, but I still did what they requested.
So how strict are they? For you to start on their care pathway they will want you to explain your gender dysphoria. This does not mean you have to describe that you knew for the age of 4, as this is just not true in everyone. But be prepared to explain when it did start, how you have felt at key life moments... puberty, losing your virginity, relationships romantic and social; basically how you discovered and confirmed to yourself who you really were. You are living full time, you know who you truly are now, making up experiences shouldn't be needed.
Medically I'm unsure how your bipolar diagnosis will affect you completely. But I know from every report that I have received at every single stage that they are looking for stability. Are you a functional member of society, have you got friends, support network and a job? A precondition does not exclude you from treatment, but they want to make sure that you are mentally resilient enough to survive the transition. Again you are already living full time, and I assume you are fairly stable in your life now, that is all they are really concerned about.
When you go take the evidence that you have acquired on your real life experience to date. ID's, bank details etc are all good primary evidence... and you will need to show this evidence again when you apply for your gender recognition certificate.
Oh regarding dress, when I went I only wore a dress once; pretty much all the other times I wore jeans and a teashirt, although always with nice shoes and accessories; wear what you are comfortable in they are not looking for gender clichés. The appointments are informal for an hour, they are like a chat, with the occasional hard/awkward question thrown in. I pretty much didn't shut up once I got going and when he wanted some other information he redirected the conversation. After you have had 1 appointment, you will have one with two specialists, and then another appointment with yourself and someone who knows you. This is unless they have changed in the last few years. After the three appointments they discuss your case as a team, and decide whether you are an appropriate case for medical transition.
The time until HRT will be the point they accept you on their care pathway... so 6 to 9 months I think, the other precondition is that you are full time ( so no problem there!).
I hope that has helped a little. I know they were a bit broad answers ,so if you need anything specifically ask or PM me.