The reason behind this is simply that the USA is a vast country, 75 times bigger than England. It has large areas that seldom ever see tornadoes and it is this sheer size that acts to dilutes their density.
Also tornadoes are only reported if they’re actually seen by someone – this means population density is important too, and England has a high population density, with lots of eyes on the sky.
In contrast the infamous ‘Tornado Alley’ in the US has a low population so it is likely that many tornadoes here go unseen and therefore unreported.
If the US state of Kansas were a country, it would have a tornado density of 4.5 per 10,000km² which is more than double the figure for England.
The smallest country in the world is Vatican City – if there were ever to be a tornado recorded here, then that one storm would catapult it to the top of the tornado density league.
So tornado density statistics can say about as much about the size of a country, as they tell us about the likelihood of seeing a twister.