Places in mental health secure units are thin on the ground, Jingle. She will be assessed and recommendations made for treatment, and hopefully prioritised for a MH bed. The consultant forensic psychiatrist who will oversee her progress in prison can move heaven and earth to get prisoners like this a place, but it's always at the expense of others who get shifted back to prison to make the space, if there are none who can be discharged or released into the community.
MH patients in medium secure mental health units, and special hospitals like Broadmoor and Rampton, also have rights and can fight attempts to move them back to prison. Having visited severely mentally ill prisoners in their cells, because they have been segregated to protect themselves and others, I have been horrified at the terrible state some have got into, despite best efforts of prison officers and medical staff, and I have watched whilst forensic psychiatrists have tried to mobilise anyone they can bring in to try and find an immediate place, often taking days or weeks before a place is freed up. By rights, this country should have more secure mental health places than prison cells.