I do think that voting pretty much entirely on a "single" issue would be a little bit irrational (although not unreasonable per se because it's obviously up to you to decide what's important to you). There are so many things that affect women's welfare and vulnerability to abuse, and speaking very personally, I don't see the trans issue as being the most significant of these.
I'd be inclined to have a look at which party is more likely to fund domestic violence support, sentencing reform, tackling the causes of crime, childcare, funding development budgets to help women in developing countries etc. etc.
Although that's hard when Labour won't say what their spending plans are! You might need to wait for the manifestos.
If the major parties don't have much between them in terms of wider policies to help women (possible, given labour's various approach) then you could use your "single issue" as the main decider.