No, this isn't a full scale invasion on Europe but rather a single daily 777 from London/Heathrow to Brussels. Brings back memories of when Pan Am and TWA used to fly all around Europe, doesn't it? But the reasons here are very different. Does United think there's a big market for those going between Brussels and London? Not likely. I'd guess it's all a slot game.
See, the winter schedule is showing a reduction of flying to Heathrow, so I'm guessing United doesn't want to risk giving up its slots by not using them. So instead of continuing to fly money losers to the US, they'll simply fly less of a money loser to Brussels until demand picks up enough where they can use that slot for something good.
The plane spends a long time in Brussels as well. It arrives at 335p and doesn't leave again until 605a the next morning. That's an early wake up call for anyone wanting to take that flight west. But ultimately, this is more of a placeholder than anything else.