surprizingly it's actually safer than cycling on country roads! Most drivers in London are polite and vigilant whereas they will actively try to kill you in the country.
Cycling (and driving) in the city is safer in part because the cars aren't going very fast. They aren't going very fast because they can't. There are simply too many obstacles in the city, whether speed humps, narrow roads, crosswalks, four-way stops, or heavy traffic. I believe this is why DC has the lowest traffic fatality rate in the US [1], despite the near-universal belief that driving in the city is dangerous.
Most drivers on country backroads where I live (Cotswolds, UK) are safe and respectful. The dangerous places are provincial towns/cities - worked and cycled in a large Staffordshire town for six years, it was horrid - and rural A roads, but any decent cycle route planner (ie not GMaps) will keep you off the latter for the most part.
The dangerous part is a car coming over a hill or around a bend and not seeing the slow cyclist in front of them until it's too late. Not that the drivers are actively looking to injure the cyclists, but that the speed differential between bicycles and autos leads to terrible accidents.
