Martin, I read your post with interest.
I was amongst the first cohort of nurse prescribers so can well understand the frustration that you might be feeling!
It is likely that you are the first nurse with this new skill within your area of practice, so will have to assist your employers in dealing with this precedent.
Have you actually found out what the organisational barrier is - for example, it might be that there is not an allocated prescribing budget, with on site dispensing of a limited selection of PGD drugs?
There is no reason why you cannot prescribe alternative medications using an FP10 if you are qualified to do so, and I believe that with a well planned rationale and some assertion, that you can contact your PCT Non Medical Prescribing Lead to order some FP10 pads from Instron. You will likely have to reassure your manager that you will continue to prescribe using the in-house formulary whenever possible, as unjustified fears of rocketing drug budgets and 'loose cannon' nurses might be another reason for his/her reticence. In my experience this can take a frustrating few weeks when you try to set up a new project. but do persevere because it is worth it in the end.