The facts aren’t entirely different to R (K) v Camden and Islington Health Authority [2001] EWCA Civ 240, in which the health authority had failed to find a suitable supervising psychiatrist following a deferred conditional discharge, but had used all reasonable endeavours and so had acted lawfully.
Buxton LJ thought the patient might have a remedy in the European Court of Human Rights against the State. Sedley LJ doubted this but said:
In the present case it was doctors working for the North London Forensic Service, a limb of the Enfield Community Care NHS Trust, who formed the professional judgement that they could not responsibly undertake Miss K’s supervision in the community. If such an attitude were shown to have been adopted less as an exercise of professional judgement than as a closing of the ranks against an unwelcome decision of the Mental Health Review Tribunal, the courts would not be powerless to intervene.
If you could get a deferred conditional discharge, maybe with the help of independent evidence if you can’t persuade the treating team, then you could consider a judicial review depending on how much effort area A has made and the reasons for area B’s refusal.
I’ve tried to answer without reading the whole judgment or anything else, so I’m sure there are other points to add.