England is not a nation
You're bold!
One answer would be: England is a member of FIFA and therefore counts as a "nation", as does the occupied six counties Northern Ireland and the Farøes.
On the other hand, "unrecognized" nations' flags can cause just as much trouble. The St George flag is often used by right-wing gangs in England (eg EDL). Imagine raising the flag of Kurdistan while playing football in Turkey, or of Slavic Macedonia when in Greece, or the Confederate flag in a black neighbourhood of the USA, or - closer to home - the Tino Rangatiratanga flag if you're playing against the Don Brash Fan Club XI.