Gender on application Watch
I don’t think it really matters which you put down most of the time. I believe they only ask gender to make demographic statistics of the UCAS cycle.
Although your specific unis may have scholarships aimed at a specific gender.
You can put the gender you identify as. I think the gender you put on UCAS is what the universities you apply to will mark you as so it makes sense to use the gender you want to be recognised as rather than having to change it with the university you go to when you start.