I was always under the impression that with or without surgery in the UK we are allowed to use the toilets for the gender we identify with. I have always used female toilets pre and post op and I have never had any difficulty.
I think there is a fine line between cross dressing and actual transsexualism here. Where cross dressers are cis-gender and have no wish to actually change their sex and transgender people are those that wish to undergo (or have undergone) sex-reassignment surgery.
The law would obviously state for cis cross dressers to use the toilets for their birth gender. However I believe you should use the toilets for the gender you identify with. I had a few toilet issues in college but after the college looked into the situation they informed me I was to use the female toilets because I had started transition (Name Change Deed, Hormones etc)
I don't think you should be using the men's if you have already started the ball rolling and when you have the ball rolling
nobody should stop you or tell you to use the other toilets. It comes under the discrimination law for transgender people I believe.
As for those who are fine with your gender change but insist on calling you 'he' just call them out. Say politely and firmly 'My preferred pronouns are 'she', 'her' and 'hers'' If they fail to do so, just misgender them until they get the message. And be persistent.

Sorry I couldn't help with the
actual laws but I'm pretty sure you should just do whatever the ->-bleeped-<- you want cause everyone else does

Much love, Saff