The Welsh are the descendants of the Brythoniaid (British), who's kings were killed by the invading Anglo Saxons (English) in the 6th Century. Unfortunately. Although the massacres inspired some incredible poetry.
The term British now ironically enough represents a colonialist English mindset that the Celts generally don't feel represented by. There's a huge cultural difference between England and Scotland, Ireland, Wales and even Cornwall. Yet, everyone gets along and there are plenty of shared experiences and cultural references.
Just some historical context that is often forgotten when discussions of "Britishness" crop up.
I think it highlights that Mo Farah being a Somalian, Londoner, Englishman is free to represent the country that he grew up in and where his dad comes from and shouldn't have to justify his existence all the time. To represent Britain means allsorts of things depending on the individual's cultural identity, family ties, language etc. and it'll mean something unique to a Somalian refugee who was educated in England in a multicultural society, running through the mudbath xc courses of the English champs.
I think it's all really interesting and it's so short sighted, narrow minded and boring to use insults like "plastic Brit".