England, Wales, Scotland, and Northern Ireland are constituent countries in a larger country called the United Kingdom. When dealing with international affairs, you typically want to be talking about the UK (unless the subject is sport), as that is the sovereign state. But for domestic affairs, it’s often split in ways that are confusing if you don’t understand various bits of history stretching back about a thousand years.

For instance, England and Wales share a legal system, but Scotland and Northern Ireland have their own. There’s bunch of special things about Northern Ireland and its relation to Ireland. And all of the constituent countries have their own parliament / assembly except for England (the parliament in London is for the whole of the UK where MPs for all constituent countries can vote).

In this particular case, the constituent countries have their own health systems, so it’s normally England we would be talking about, not the UK.