Stonewall is associated with and supported by the feminists.
The feminists, far from seeking equality, work toward greater separation and a reconstruction of western society to their liking. That includes a destruction of western femininity.
The feminists will never support transgendered people, especially MtF, because they seek to be feminine.
Worse though, the feminists have separated themselves from the struggles of women in non-western countries. Theirs is a lip service acknowledgement which includes dismissing FGM as being of no more importance than western women's fashion.
Sadly, for the British left, they can't oppose the nazism of the feminists for fear of incurring the ire of Stonewall UK. They can't risk losing the support of Stonewall UK.