English is not an ethnicity. Its a nationality and a language.
You'll find the UK "white" is a complex mixture of ethnic backgrounds including Britons, Angles, Jutes, Saxons and Normans. The language also reflects these roots.
And then as a result of the Empire, Colonies and Commonwealth - the UK has had significant immigration from pretty much every country in the world. These have all left their imprint on the genetics and culture in general. Just look at food - The British national dish is Tikka Masala ffs. They drink wine from New Zealand, beer from Holland and cheese from France. Carribean, Hong Kong and Ethiopian restaurants can be found everywhere. Those of Chinese ethnicity make better chips than most white fish and chip outlets. The national football song is Vindaloo. (As a great example, the USA football league started chanting this at games but for relevance had to change the food to "BBQ")
The country, especially London, is the melting pot of the world. The idea of a standard British or English ethnicity is laughable.
If anything, Mo Farah is the perfect example of a modern day Brit. Roots from far and wide but calls London "home".