So you don't understand what it means to be transgender, you don't understand UK law, you don't understand that giving a minority the right to a dignified life has no tangible effect on females.
Firstly, UK law has been clarified and it means exactly that the terms male and female; man and wan refer explicitly and exclusively to.people born into that sex.
Secondly, if the term female is changed to include males, then the tangible effect on women is that the word becomes meaningless and there is no single easily understood, unequivocal signifier for actual women. If something can't be identified specifically in language it ceases to 'technically' exist.
In much the same way as in other languages there are words to signify things that we don't have words for in English. So we simply never refer to them, acknowledge them or consider them in any meaningful way.