You were in the bus lane starting to enter just before the bus shelter. GSV measures this as 12.8 yards in the bus lane, but I would estimate only about 6.7 yards fully in the lane, which I would think is well within de minimis considerations, but councils, especially London councils, look upon trival contraventions like this as their bread and butter. So if you do want to fight it, you'd have to take them to London Tribunals.
Anyway, submit reps on the basis above, (de minimis), and post their response when you get it.