I just had a quick look at extant guidance for Prisons on the Care and Management of Transgender Prisoners.
I've never looked at this before so by no means fully versed with how this works, however, it appears that there is already a presumption in favour of housing self-identifying transwomen (no GRC) in the female estate.
The guidance states that there may be a minority of cases where this presumption will not be supported:
In the minority of cases where that is not possible, the reasons for departing from this starting presumption must be clear, explicit and made known to the person they affect, especially when it involves assigning someone to a male or female prison.
The majority of such cases are likely to concern transgender people convicted of serious offences, where both public protection and the best interests of the person themselves may not be compatible with the general presumption described above. However, this will not be true of all transgender serious offenders and it is important that policy for the majority should not be founded upon the highly complex considerations raised by a minority
So if a MTF prisoner who self-identifies (no GRC) and who wishes to move to the female estate can already do so under current prison policy, what is really going to change if self-id is enacted under the GRA?