Until the superb Numberwatch crunches the numbers, here's my take on the risk.
The strain that has emerged in Mexico is the same as the H1N1 strain that caused the 1918/1919 pandemic, so let's look at that pandemic.
A third of the population were infected. Of that third, mortality rates were somewhat greater than 2.5%. Mortality was concentrated amongst young adults; children and older adults suffered a lower mortality.
In this pandemic an absolutely worst case of 750k deaths in the UK has been forecast - about in line with the 1918/19 infection. Except they didn't have Tamiflu, or an NHS, or all the things we have now. We also live less crowded lives.
And I shall cough alarmingly at the first idiot I see wearing a face mask on the train or on the street.