I've spent a few minutes looking up stats, using similar terms to those described. Not long, admittedly.
In those few minutes, I did come up with some US stats from a source that looks reliable that match yours. I've not been able to find any for the UK for that exact claim, though.
What did come up a few results further down in response to the search terms was the following, which may help you to understand why some females* *may want female-only helplines in some situations.
Comparisons of official MOJ statistics from March / April 2019 (most recent official count of transgender prisoners):
125 sex offenders out of 3812 women in prison = 3.3%
13234 sex offenders out of 78781 men in prison = 16.8%
76 sex offenders out of 129 transwomen = 58.9%
*Note: I'd like to use the word "women" in my own comments above, but defer to your sensitivities by accepting the rather dehumanising use of "females" instead. The UK government stats above do use "women" to describe "females".
I guess that the people compiling the stats felt that we do still need a word for adult human females, if information like the below is to be collected, recorded and understood.