What is so special about this six week group therapy program that makes them want you do do this first? Is there a personality disorder focus? Is it a particular doctor? How have you done in previous inpatient hospitalizations?
I guess I'm a little torn here...on one hand, I can certainly understand how getting into the right treatment makes all the difference in the world. My daughter was in treatment for years before finally getting the right diagnosis. Once she got it and had the right skills training, everything turned around for her.
On the other hand, I can also possibly see why your clinicians may not want you to be inpatient. I'm not sure what your diagnosis or background may be, but they have found that some people with certain personality disorders actually fare worse in an inpatient setting than they do outpatient, once the initial crisis has passed. They become acclimated to the hospital environment and in some cases, act out in order to show how much they still need to be there. I am in no way saying that is the case with you, just something to consider.
I am not sure how the NHS works, as I am in the US, but is there some way you can get an advocate? Someone who is not a family member or a clinician, who can be there to support you and help you navigate the system? Who can look at your case and be an objective voice to help you and your pdoc come to a better understanding? An advocate could also help you talk to your mother about the insurance issue.
Can you consult with a pdoc at the private hospital to see what they think about the inpatient option? Again, I am unfamiliar with the NHS, so I'm not sure how the private hospitals and the NHS clinicians interact. But could the private pdoc put in an appeal to the NHS?
Sorry if I have more questions than answers. I think this is a very complicated issue and there isn't a simple answer. The six month option may be the NHS's way of getting you out of the inpatient setting to help stabilize you, or it could be a cost-cutting measure. The inpatient stay may nor may not be what is truly best for you. The dedicated DBT program sounds like a good option. Isn't that what the personality disorder team would provide? Not sure why they don't just start you there. I've been in a similar situation, though, where I was denied access to a DBT program because I hadn't been stable long enough. It really didn't make sense to me at the time, but it sort of does now. I was also put into a group therapy program, but it was with a personality disorders focus, so it was still beneficial. It just wasn't the dedicated DBT training. (If it's any consolation, at one time they thought I did have BPD, but I no longer fit the criteria and haven't for years now. It can get better.

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Here's a link that may help. I'm not sure if this is something you are eligible for, but it's worth a look:
http://www.dh.gov.uk/en/Healthcare/M...hAct/DH_091895
I hope you are able to get the care you need.