Tolley, for one you are not on your own, secondly there is no reason really why you should be denied the rifle, providing you have land or have booked to stalk larger species of deer, thirdly you should be able to take fox whilst out stalking with just about any calibre.
Best of luck, dont give in with the FA unit if they insist, I know of more than one person recently who eventually got the calibre they wanted through putting a sound waterproof case forward.