GP stopped my ongoing shared care agreement without warning, is this allowed?

Hi, I have had a private ADHD diagnosis and was lucky enough to get my GP to agree with the shared care agreement. I have been getting Elvanse prescribed for 9 months now by my GP. My prescription got rejected out of nowhere without any notifications or emails from them. I managed to get an appointment with them 2 weeks later to have them explain what was happening, but not without going through some horrible withdrawals first. My GP told me that they stopped the shared care agreement because they don't do them anymore and don't allow mixing of private and NHS treatment.

I am wondering if they are just allowed to stop prescribing them to me without warning or consulting my private consultant. I am also wondering if they are allowed to do this at all after agreeing to the shared care agreement for this reasoning? It seems very strange that they can out of the blue go like no we don't do ADHD private prescriptions anymore after taking over the prescription through a shared care agreement months earlier. I can imagine that they would do that with new shared care agreements, but i can't imagine they can just stop the pre-existing ones without notice.

Before I went for a private diagnosis I discussed with my GP if they would be able to agree with the shared care agreement and they said yes, and i have spend a lot of money getting this diagnosis, only to be told that they don't anymore 9 months after agreeing to the shared care agreement. I am kind of lost to what to do now. Any advice would be much much appreciated and thank you for reading this!