The doctors are simply saying that they cannot prove by the means available to them that you have NF2. There is no genetic inheritance and the blood test is 60-80% accurate at best. NF2 cases do not necessarily have to be inherited because the rate of spontaneous mutations is 50%, that is, 50% of cases appear out of nowhere. Furthermore, the blood test is not sensitive enough in your case to pick up the mutations you do have. Either it can't or your mutations are very rare and they are not being tested for. Compounding the issue is the fact that NF2 has a very high rate of the mosaicism, i.e., mutations are in some cells but not others. NF2 mosaics have many, many symptoms of the disorder but their mutations cannot be found in the blood, presumably because they are not there. Mutations always exist in the tumor regardless of whether or not they can be found simply because no tumor is normal and some genetic atypia has to be present for the tumor to grow.
In the US, you would be diagnosed with NF2 whether or not it can be currently proven because it is by definition "bilateral acoustic neurofibromatosis". Spine scan will be a very wise idea.
There is another NF2er from UK - Tony. I forgot his exact user id and we have not heard from him awhile. I miss his insight and sense of humor. Maybe you could send him PM.