I asked this to a recruiter recently mate and the answer I got was that they access each person individually. So your GP would send your info to the ADSC and if they thought you were suitable for training etc, you should be fine
yeah you can get inserts as feet can even flatten out during training. i have a mate who did the "them" course recently - showed me his feet and they've flattened out (he runs alot anyway) and so he has been given inserts either by an army doc (a chiropodist) or a civvy one and although they hurt for a while, the pain will stop and they will eventually train the arch to be an arch again. Its muscle we are talking here.
If its more serious and skeletal I cant recommend anything as its not my area of expertise. I just know the inserts can work to train your feet back into shape.
yeah your right, tyhey can flatten out during the course of your career, i have been wearing the insoles now for about 2 years, they do help a little but not a great deal. however they have not trained the arch to be an arch again they are still as flat as ever.