Not quite. If you go in to the advanced options, you can check a whole bunch of other information to appear on your detonation. You want to add "Minimum radius for 100% probability of no burn" and find out that you've still singed Brighton.
I dropped a theoretical 100,000kt bomb on Croydon...
You know, kill it with fire and all that, well...
The thermal blast (big orange circle around the rest) stretched from Flitwick to Eastbourne, easily burning Sussex to a cinder. I wouldn't recommend it if you should ever happen to find...