No one has disputed Brianna existed, don't be ridiculous. Point out one post which said 'Brianna didn't exist'.
I'm sorry you don't like the fact that not everyone has to believe what you believe, but currently in the UK that's the situation. There is no evidential test for being trans so yes, it's a belief. Sorry you don't like that fact.
How about you have some respect for Brianna, instead of deciding that you know who Brianna was or what Brianna believed? But you won't do that, because you're too busy using the murder of a child to push your ideology.