We could be charitable, I suppose, and speculate that they couldn't find an English flag in the local shops. But it's more likely that it's partly a throwback to, say, 20 years ago, when the flags were less differentiated - that World Cup use of the Union Jack as the English flag was pretty widespread. I think there are probably a lot of English people who would see themselves as English but who also identify with the Union Jack as their flag - after all, it is far more common than the English flag; it is the flag for the place where they live. The Onion Jack has more than one layer
Still, you'd think that whoever was waving the Union Jack would have twigged that it wasn't the 4 nations cup...