Based on what? Nothing points to a higher mortality rate - but some countries are admittedly not testing a lot. Even if you quadruple the estimated mortality rate, the UK numbers would still be under-represented.
Lower bound: Deaths/(total cases)
Upper bound: Deaths/(deaths + recovered)
Some media in France tend to focus on the upper bound, and reports numbers in the 10-30% range, so the country is very open to very severe measures. The US tends to be optimistic, and everyone plays it down, so the virus is spreading like wildfire.
People in the US tend to look for every silver lining, preliminary study showing a potential breakthrough, and any reason it might not be so bad. That's a bias which has led to this not being taken seriously enough at any point.
I kind of treat the WHO estimates (1% with ventilator, 6% without, and 3.5% median) as just that: best-available estimates. It might be much, much worse. It might be much, much better.