The NHS has the world's largest deployment of Microsoft Exchange server. I believe it is somewhere in the region of 850,000 users. NHS England has 1.2 million employees in total, if you include NHS Scotland and NHS Wales, it is 1.4 million. Northern Ireland has its own health service which isn't called the NHS.
They are the world's fourth largest employer, and the three largest - Walmart, People's Liberation Army and Indian Railways, don't have as many people who would use email at work.
So anyway, we are looking at a ransomware demand of at least £200m, which the NHS certainly doesn't have as spare cash.