As a Heathrow-based 747 pilot, the conversation in the UAL flight deck was probably along the lines of
"He's a bit slow clearing isn't he?
"Sure is....how low do you want to go before hitting the go-around button?"
"Well, I'm happy to leave it to 100 feet, but let's see what the Tower says"
At which point normally the Tower controller tells us to make a go-around, or we tell him we are/aren't happy with the way things are going.
My point being that it's not a drama, it's something which we (the pilots) and they (the controllers) can see developing, and it's just a question of when to call it.
In the company I work for, the go-around is ALWAYS included in the pre-descent brief. That's because if it should happen, no-one is caught by surprise (always a Good Thing in our profession.