Is Scotland a nation or a country?Watch
It's a country because that's the word we use for the different parts of the UK, even though the UK itself is a country in a different sense.
But it is not a Kingdom because the United Kingdom isn't pluralised. The whole thing is one Kingdom, and the Acts of Union that brought England and Scotland together were quite explicit that they were being merged into one Kingdom.
As for a nation - well, often nation is just a synonym for a country. However a real nation comprises a group of people - it wouldn't be Scotland, but the Scottish people, that would be eligible. As for whether it is or not - that's entirely subjective and based on your view of what is distinctive enough. It's reasonable to say it is or it isn't.
For my part, I don't believe nations exist. As a sociological concept, they fail to recognise the different cultures and identities in the modern world. They depend on a fiction of uniformity and homogeneity. In many cases, attempts at defining them simply cause conflict: Ireland, Israel/Palestine etc.
England, Scotland, Wales and NI are separate nations which come together as a country called the united kingdom. Thats why we can enter certain international competitions as one, for example, the olympics, as that's between different countries. I think the same principal applies to places such as Trinidad and Tobago. Each of them is a nation, but i'm assuming they have one government and together form a country.
Although I'm not sure why we then have to enter the football world cup separately to scotland etc, but Trinidad and Tobago are together for it
Really it's a bit of an anomaly that England, Scotland, Wales and Northern Ireland have separate teams for some sports. It's because most of them started in the UK so the first "international" competitions were between the different parts of the UK.
FIFA actually have a definition of "country" for the purposes of their membership criteria. Under that definition the UK is a country, and not the separate parts. Normally each country can only have one team, but the UK is an explicit exception to that rule which is why there are four separate UK teams. I don't suppose the people in charge of FIFA would mind getting rid of that exception, but there would be huge outrage in the UK if the separate teams ceased to exist.