It kind of depends on a number of factors. If you snap a photo of an airplane and pilot at a public exhibition, like an airshow, you can safely use it commercially without legal liability. If the pilot is the subject of the photo, ie a closeup of him in the cockpit with his smiling mug, you probably still could use it commercially, but it is more of an ethical issue. Personally, I would want to get permission from the person before selling usage rights for advertising or something like that. Would you want to suddenly see your face on an ad for something? Obviously, selling the photo to a magazine or aviation periodical in another story, especially if it is covering the event that the photo was taken.
There are places where the frown upon you taking photographs, like power stations, water treatment facilities, etc. However, obtaining prior permission from the location for the photo and having a letter to that effect on your person can help alleviate some misunderstanding. I know that is not always practical. And the key is to be smart about it. Taking a snap or two of an electrical tower is one thing. Hanging around for an hour walking around it and taking photo after photo of it is going to arouse some suspicion.
For you, Lee, you have a pretty good out. Throw out a little cockney slang and get enough out that you didn't know and they will most likely just smile and move on, or tell you to move on.
You can be on public property and still get harassed, but chances are if you are polite and respectful, state what you are doing there and why, it will usually be no big deal. Remember that security people and police are doing their job to protect all of us, and if they are checking you out because of that huge lens, they are just making sure you aren't doing something stupid (read illegal).
I got stopped at Camarillo once when I walked all the way down to the end of the field and back with my camera, snapping shots. The guy asked where I started my walk. I showed him and was polite and he just asked that I stay in that general area. No harm, no foul. The one thing you should do is know what the boundary lines are and not cross them. If unsure, don't get too close. There is a yellow line at the taxiway that I can go past, but the second I cannot. I normally stay behind the line I can cross so there is no mistake that I may be pushing the limit. Plus the area between the lines is for cars and trucks. You may not hear the vehicle approaching over the sound of the aircraft. Plus there is always the possibility that if I am looking at something interesting to photograph, a driver might also be looking at that and hit me.
It's pretty much about common sense and respectfulness.