One large light that dimmed and got brighter as it changed direction, and two small red lights that seemed to move around the larger one. This was at high altitude so it made seeing the shape impossible.

What I thought was an a high altitude jet airliner crossing North West above London it then came to a stop, headed East and then went back forming a triangular shape. The reason I cant imagine it was a plane is that it didnt have a turning circle of any kind, it just instantly changed direction. It was also too high for a helicopter and definitely moving too fast.

After watching this for a few minutes or so I went to get my house mate to have a look, and in the short time I took to get him the object had gone, any plane would have still been viewable in the 10secs I took to get him.