The speed limit in London is at 20mph primarily due to safety, not emissions concerns. It takes approx 2x the distance to come to a complete stop from 30mph than it does from 20mph.

For the majority of journeys in London, you're sitting at a red light, or transitioning to the next red light. Not a lot of opportunity for sustained 30mph travel. Accelerating up to 30mph so that you can travel the 300 meters, and then stop for 3 minutes serves no benefit to you (because your journey is still predominantly waiting at traffic lights), but reduces safety for you & everyone around you.