Thirdly, Jane is actually right that there is no automatic right to go into a female prison even if you have a GRC and there won't be once the law is changed. Heather and anyone else who goes on media needs to be absolutely 100% bang-on with what they are saying.
I'm getting bored of posting this but I will continue every time this comes up.
Prison instructions for trans prisoners can be downloaded here - scroll down to '17/2016 The Care and Management of Transgender Offenders'.
The prison service go by legal sex, first and foremost. They're not allowed to ask to see a GRC but they can ask for a birth certificate (the sole remaining thing a GRC is good for). If a prisoner has a female birth cert (original or obtained via a GRC) they will be housed in the female estate unless they present an exceptional risk, in which case they may be moved to a men's prison, purely because this is where the facilities are. They will still be housed separately from men and will be accommodated in line with PSI for female prisoners. TW prisoners with a GRC are treated as female in all circumstances.
There is case law directly referenced in the prison rules linked above that says that even if a tw is in prison for attempted rape, if they have a grc and hence a bc that says 'female', they must be sent to a women's prison. The bar is very low as we have seen in various cases that have made mainstream media.
Only for trans prisoners without a GRC - i.e. those who are legally male - are decisions made on a case by case basis. And the Transgender Case Board are no fools, which is why all those self ID trans sex offenders are still treated as male prisoners in the male estate.
Changing the GRA to a self-ID process would allow all of them to be treated as female. The vast majority would have to be moved to the female estate, with only the few exceptionally dangerous ones remaining in the male estate, but housed there as female prisoners.
Today's article in the DM contains quotes from prison service staff expressing their concerns.