Witnessed an elderly cyclist get overtaken twice, by the same coach driver, quite dangerously. What should I do?

Earlier I witnessed a coach overtake (at around 35mph on a 20mph road) an elderly cyclist. It then came to a stop at a red light. The cyclist moved ahead at the light and after pulling away the driver did it again. Both times not leaving a lot of room and both times going well over the speed limit. I saw the old guy visibly wobble each time. He was on an electric bike and not moving particularly slowly. I clocked the registration of the coach but it was unmarked (or I couldn't see the company name). What should I do with this information? If anything? I'm concerned the driver might do some serious damage to someone in the future.