At galas, open days, it's going to happen. Not too much of a problem in this day and age of digital photography, just delete the spoiled photo and retake. Stand there long enough, and you'll get the shot you want. Just accept there are going to be a lot of people there, and they don't have eyes in the back of their head.
The big problem arises when you are doing photography in other places. I have had people to stand in front of me, start a conversation, knowing full well I'm aiming my camera at something. This may be a British problem.
Taking photos of buses in Switzerland, people were very considerate. Some people actually stopped walking if they couldn't pass behind me, until I took the photo. On one occasion, a bus was coming through a narrowed section of road, a car had to give way. The driver saw me with the camera and waited a distance away from the Stop Line. A wave and thank you from me, a nod from him, after a good photo. People at railway stations were also very considerate.
Slightly off topic. I did once clear a bus load of druggies. There was a bus enthusiast taking photos of the buses on the town centre main road. It's a one way system so photography is generally easy. Driving down the hill to the town centre the bus was now full with over half the passengers being the local drug mob. One man sitting close to the cab was getting a little fed up with this mob mouthing off. He knows me by sight and knows I'm interested in transport. "Driver." He suddenly said. "Do you know there was a bus photographer in town earlier? Do you know him?" I looked at him for a second then at the local drug mob. "He's not a bus enthusiast. He works for the Benefit Agency!" I said in a loud voice. The bell rang at the same time there was silence. The mob got off the bus several stops early. They were not seen for the rest of the day.
