England is a country just as Nigeria, and its capital is London just like Abuja is the capital of Nigeria.
Great Britain is a collection of 3 countries which came together as one country and has its capital as London. Those 3 countries are:
1. England,
2. Scotland and
3. Wales.
When you include Northern Ireland to these 3 countries, they become United Kingdom (union of four nations) Viz:
1. England,
2. Scotland,
3. Wales, and
4. Northern Ireland.
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That is why it is referred to as the United Kingdom of Great Britain and Northern Ireland.
Collectively, it is taken as a country. And when this is the case, London is its capital.
Note however that the individual nations all have their own respective capitals.
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Accordingly, the capital of
Scotland is Edinburgh,
Wales is Cardiff; Northern Ireland is Belfast;
England is London.
Ps: Northern Ireland is not to be mistaken for the Republic of Ireland. The latter is a separate country. Its capital is Dublin.
Thank you.