What is the capital of England?

London is the capital of both England and the United Kingdom.

https://en.m.wikipedia.org/wiki/London

May I please add that although London is indeed the capital city of England, there are three other countries that comprise the United Kingdom. These are Wales, Scotland and Northern Ireland.
The capital of Wales is Cardiff, the capital of Scotland is Edinburgh, and the capital of Northern Ireland is Belfast. So technically speaking, the United Kingdom is these four nations united, and therefore London can't possibly be the capital of the UK.

Scotland and England are "constituent countries", Wales is a principality and Northern Ireland is a province.
As Gnomon says, London is the capital of both England and the UK. Capital cities are almost always capitals of at least two areas at once, e.g. Rome is capital of Lazio and Italy, Edinburgh is the capital of Lothian and Scotland and so on.

London is not only the capital of England but the entire UK.

https://en.wikipedia.org/wiki/London