How can they (Stonewall) support LGB people when they say homosexuality is same gender attraction? I honestly don't get it
Because "LGB" is about same-gender attraction, according to Stonewall. So they support "LGB people" - ie same-gender attracted.
(I guess if you're niche gender 37, you get in under the "T", regardless of whether you're attracted to gender 37, anything apart from 37, or only to gender 45 specifically. The LGB bits are only for the boring "male" and "female" genders).
If you think same-sex attraction makes more sense, you probably need to look to new groups like LGB Alliance and Lesbian & Gay News.
And this of course means Stonewall are not the obvious people to go to as experts on the Equality Act, which specifically protects actual sexual orientation.