c. The argument for trans rights and the GRA is similar: why should people not be allowed to change their legal gender?
Because they're identifying into another group's protected class.
So not at all similar.
Trans people were also prevented from various other things like being buried as their gender (which seems frankly cruel)
How does this work in practice? What happens if you are 'prevented from being buried as your gender'? Does the vicar misgender you throughout the service? Does your 'deadname' go on your headstone?
Or is it just that a death certificate records your sex in the same way as a birth certificate? There's even less justification for changing death certificates than there is for changing birth certificates because you're no longer even around to get upset.
... legally can't impact on cis people except in a tiny number of circumstances
Prisons is really quite a biggie though isn't it? In the JR against MoJ, female prisoners' articles 3, 8 and 14 rights were discussed.
Also the impossibility of using the occupational exceptions in the EA. Many individuals who need to use this exception are disabled people with daily intimate care needs. This group is not tiny in number.
the right to update records of birth and marriage
Is this a right? What sort of right is this? Do we all have that right?
I'd quite like to change the date of birth on mine because increasingly I identify as ready for the state pension. Maybe I could change the place of birth and gain citizenship of another country. Perhaps I could change the names of the people who are recorded as my mother and father and secure a large inheritance If I identify as next in line for the throne can I put down the queen and prince philip?
Why is the answer to all of these 'no, don't be ridiculous' but when it comes to sex this is somehow completely reasonable and anyone who questions it is a bigot?
All the GC claims seem to be that as some of the presenting discrimination has changed over time, that the fundamental reasoning that the government was breaking human rights law in relation to trans people goes away but that's not how Human Rights Law works; it's a right you can expect of your government and it's the government's job to prevent discrimination against people in minorities because they don't do a lot of good otherwise.
You don't appear to have the first clue about how human rights work.
The GRA in itself is not a human right. It's not a human right to change the details on your birth certificate. The GRA was an extremely fudged solution to legitimate human rights issues identified by ECHR, all of which are now better met by more recent and more coherent legislation.
Does it not tell you something that trans people gained the right to marry someone of the same biological sex in 2004 but everyone else had to wait a further decade?
There is no justification or obligation to keep the GRA in perpetuity just because it was once upon a time a handy workaround for a homophobic government.
So I ask again - what actual rights would be removed from trans people if the GRA was repealed?