1) There's no such thing as road tax. Many cars pay zero emissions duty so that's that point dead in the water.
2) I cycle every day, and have just started cycling through central London twice a day too. The cyclists are the least of the problems, the drivers are just appalling. Especially the taxis, who will just carve across lanes and stop on double yellows (or right over a cycle lane) without indicating.
No-one on the roads is perfect (does anyone drive like they did in their lessons?) but drivers need to remember that if they collide with a bicycle, there's only one winner. Yet it continues to happen, day after day.