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.
Sadly these days, sickly folk can fly round the world on cheap for a fraction of their benefits.
Whilst I have faith in modern medicine and society, I'm not going to rule out the possibility of The Cassandra Crossing re-enactments on aeroplanes.