It would depend on how you defined "safer". There's probably a greater likelihood of a collision in a traffic congested city like London due to the sheer number of road user interactions, but as the vast majority of roads are limited to 30mph or 20mph (and motorised traffic can often be going considerably slower than that) survivability in the event of a collision is increased (you're far more likely to survive at 20mph). However, this is not true of HGV and cyclist collisions.
Once traffic speeds get above 40mph, in the event of a collision between a cyclist and a car, the grim reality is that you will be far more likely to die than survive.
More cheerily, you're likely to live longer if you cycle, so just ride.