Maybe I'm missing a huge point,
Yes, you certainly are. If you had RTFT, you might know what you're missing, but evidently you didn't bother.
So here it is: female people are fully realised humans who have rights under UK law. Now your comment strongly suggests you either don't understand the principle of female people having rights, or you don't agree with female people having rights, but thankfully our rights do not depend on whether you or a male person decide we get to have them or not.
I just cannot see what the issue is if someone feels and openly identifies as a woman, they are trying their best to fully embrace being a woman,
I don't believe in your quasi-religious beliefs of people being able to change sex. I am under no obligation whatsoever to share this belief or to surrender to this frankly antediluvian notion that a male person wearing feminine-coded attire is "fully embracing being a woman". Do you even hear yourself? Women are not a thought in a man's head or a costume for him to wear. We are - to repeat myself - adult human females and fully realised human beings in our own right. There is no way for a male person to "fully embrace being a woman" that does not involve him embracing the very stereotypes the male sex has used to oppress the female sex for thousands of years. It is offensive and abusive to pretend that women are the embodiment of the sex stereotypes and sex role stereotypes men used to keep us unequal. I am astonished that you fail to understand something so very basic.
but another woman gets to tell them no sorry you can't use our toilets as it makes me feel uncomfortable?
Yes she does. Do you know why? Because she has rights. And her discomfort matters. It matters just as much as his and that means he does not get to ignore her discomfort with impunity.
it feels a bit like a mean girl situation 'sorry you can't sit with us' what because they weren't born female?
The only mean girl in this situation is the one who shames a fellow woman for not surrendering her rights to a man.
if this person truly identifies as female and is fully living their life as female who are we to say they can't have a wee in the bloody toilets?
We are people who have rights of our own. That's who we are. And who are you to give my rights away? Shall I consent on your behalf to things you do not wish to happen? Why not?
Because consent is not transferable.
That's If you know this person is a decent human being why can't compassion be shown?
Because including a man in the female-only toilets excludes many women from their own space. Why can't compassion be shown to them? Are they not decent humans?