FM
Unless there are signs prohibiting photography under the Official Secrets Act (ie. at Boscombe Down), there is nothing to stop photography from public property outside the fence. There is no need to seek permission. Indeed many airfields have set up positions for plane spotters/photographers with info boards.
However, there is much debate in the photographic press recently regarding the suspicion of taking images or video for terrorist purposes; it does not have to be aircraft but includes public buildings and military establishments. If the police suspect, they can question but they cannot demand you to delete or erase images (that is theft). The best rule is to act openly and avoid raising such doubts.