I asked my GP for a vitamin D test last week. After months of arguing with NHS doctors to persuade them I needed an MRI in the first place, I arrived expecting to have to argue my case again. She did ask who had recommended the test before she agreed (my Nutritionist plus a whole body of research), saying that the NHS donâ€™t routinely test vit D levels in MS patients, so I donâ€™t know whether it helped to say that a professional had advised it, or whether she would have done it anyway. Catrionaâ€™s suggestion of taking some literature with you, is a good one. I had some just in case my GP wasnâ€™t aware of the research, but I donâ€™t think she would have admitted to this anyway (and she did give a very vague response when I politely enquired)!
I also read on another recent forum post that it might be best not to mention that you are already supplementing with vitamin D, as they may then not see the test as relevant.
Anyway, hopefully you wonâ€™t have any problems with your GP agreeing to the test.
I also live in Cumbria, so am painfully aware of the lack of sunshine here!