A transgender MTF wing, would be the sensible answer.
In which prison estate, though
I don't much care which estate they are in, provided that there is sufficient protection for the rest of the population (managed by segregated recreation time, for example, which would be normal practice anyway); that there are the appropriate facilities in the estate to support all the prisoners and their various needs; and prison staff are not required to search different sexed prisoners against their will (both the staff and prisoner's will, I mean - it's important both ways)