As atbman said, filtering is generally more accepted and expected in the UK. Whether to do it or not really depends on the junction. If I'm turning left and the car in front of me isn't, I'll filter forward and position myself ahead and on the left within a clear line of sight to the driver. When the light turns green, I can then just turn left and people don't have to wait for me to get there. If I'm going straight on or turning right and there are two or more lanes going in that direction, I may filter forward and take the leftmost lane when the light changes, or just wait in line depending on the traffic density. Left-on-red doesn't apply here unless there's a green filter arrow, so if I notice that there is one and I'm going straight on, I'll position myself over to the right so left-turning traffic can pass.