Actually it is, it's called self defence and allowed in law.
There seems to be two reactions though when some lads come marching towards an England fan area looking for trouble and you can see examples of both in the videos. There's "Ah fuck, these French lads look like they've come to cause bother. Best move inside." If some local yoof comes up to you ready to take a swing, if course you can deck the little prick in self defence.
But then there's "It's the French! Quick, wreck the cafe, grab a chunk and charge towards them chucking whatever you can!"
You could class the latter as group defence, but even if they started it or provoked it, it doesn't seem to be helpful.