UK population is 65,000,000 give or take.
4% of that is 2,600,000 who will therefore catch the virus if left unchecked (no quarantining etc)
Mortality rate is 2% which gives a death toll of 52,000 in UK using your figures.
US has a population of 327.2m which gives them a potential death toll of 261,760. Again using your figures.
The Flu season has been in full swing for a few months and Covid19 is playing catchup but is far more of a threat that flu which has a mortality rate of 0.1%