All Police forces in the UK have been for some time now been either trained or made aware of 'Hostile reconnaissance'. A part of this progress is to watch for persons photographing & videoing certain locations, and I am afraid that airports & airfields are right up there at the top of the list along with Government buildings, shopping malls, main line train & tube stations.
Remember this, it is not how but when will the next atrocity take place, because this green & pleasant land will always be a target. And trust me I have been involved in one of the worst.
So get used to being asked by people in authority what you are doing and if you answer back politely you will get treated the same in return.