I was on a flight from Lisbon to London, May 27, 2012. We were in the landing path to London Heathrow airport. I had a window seat on the plane, toward the rear on the left side ("A" seat number). I was staring out the window watching the landscape go by below. I was fixated on an airfield I saw below, a line of Cessna and light aircraft parked on the field, and one small personal Cessna style aircraft flying really low to the ground below me. I would guess that we were at, or near, 10,000ft elevation. Someone could easily calculate the landing path angle into Heathrow when the planes are in line with White Waltham Airfield, and Windsor castle fast approaching.
As I was looking at the small single engine plane flying a few hundred feet above the ground, way below me, the movement of something to my right (as I'm looking out the window of the plane) caught my eye, and I saw 3 objects, way off in the distance, just specks. My initial thought was "wow, those birds are flying REALLY high!!". I thought they were birds because of some kind of grouping pattern they were in. As they got closer, I saw one of them reflect sunlight and I thought "those are birds, they're metallic!" This was the moment that they flew past the our plane and I got a very clear view of them. There was 3 metallic spheres, traveling at a very high rate of speed in the opposite direction as our airplane, and they were a little higher than half way between our plane and the ground. There were no windows, no markings, no wings, no protruding shapes or antennas or probes... that I could see. Just smooth shiny metal. The formation the objects were in was one in front, the second directly behind the first one... with a gap about 6 or 7 times the width of the objects, and the third behind the second, about a 4 or 5 widths behind, and slight to the right of the second object ("right" in their direction of travel).
Using the single engine airplane's wingspan (roughly 30ft) that was flying low to the ground near the airfield as a size reference for comparison, I would put the diameter of each object at about 18-23ft.
The entire experience lasted about 3 seconds from initial sight (thinking they were birds) until they had gone out of my field of view behind the airplane I was in. There was no noise, no heat signature from propulsion, no visible disturbance of the air, just a straight line and going VERY VERY fast.
I've taken thousands of airplane flights for business travel and am familiar with how other airplanes/aircraft appear when they fly near or past each other in the sky. In my estimation, these objects were going as fast as cruising speed for an commercial airliner or slightly faster, (600-650mph+).
Once we landed in London and were in line for immigration, I was talking with my work colleagues and said "you guys won't believe what I saw!!". One immediately replied with "those metal balls that flew past the plane?". I said "wow, you saw it too?". Then another spoke up and said he had also seen it and shared in confusion about what it was. Then a 4th one spoke up, he said he had a middle seat, and an elderly woman sitting next to him at the window hit him with her hand to get his attention and said "did you see that?", but she woke him from sleeping and he missed it.
We were speculating about what it could be. None of us had any ideas, but the one thing I thought was curious was that this happened about 10 days before the start of the London Olympics. So we thought that maybe it had to do with some high tech surveillance. But curious why the authorities would fly 3 objects directly over Heathrow airport, in the line of an active approach path to commercial airlines, and at such speeds.