I guarantee if this the victim was muslim and they pulled off some Eid Mubarak sign etc there would be a huge outrage. 24/7 media coverage, protests in front of the restaurant, 1 star yelp reviews etc..
Doubt it would go that far, but to be honest, if this was reversed I suspect it would get added to the stat about 'increasing Islamophobic hate crime'. And would probably be blamed on Brexit.
The guy should be asked to explain himself and possibly sacked, depending on how that goes, I think. Obviously he's not going to prison (people don't go to prison in the UK for even violent crimes, even killing people gets let off) but seems worth checking out what's going on with him.
Sounds like the restaurant owners are genuine in disavowing the chap. Maybe he's a new guy and a bit dim?
It's hardly a terror attack, but I would have thought that completely dismissing it could play into the Christian victimhood narrative (that the OP likes to push). Christians here are predominantly from ethnic minorities themselves, these days (some Christians seem to have a bit of a victimhood-contest thing going on with relation to Muslims, and some come from countries where the two groups take turns having a go at each other).