Its up on the petitions and activism board, its an official Government petition so UK residents only. You'll need to give your name, postcode, and an email address. You also need to click a link they email to you or your signature won't count.
No one else can see you signed, not even the person who created it.
https://www.mumsnet.com/talk/petitions_noticeboard/4859964-uk-petition-do-not-allow-people-to-have-acquired-gender-listed-as-sex-on-government-id?reply=128001793