Two wins and a draw in their qualifying matches saw Stuart Pearce's side head into tonight's quarter-final against South Korea as Group A winners.
Birmingham star Butland, 19, said: "We have come under the radar but we feel we have the players to do really well. Hopefully people realise that.
"We are here to do the business - we are not a Mickey Mouse team.
"It is understandable that the focus has not been on us. We have not played as a Great Britain team for some considerable time.
"The Olympics has solely been around other sports as far as Team GB has been concerned. But we are building momentum nicely.
"Most of the teams have had more preparation time. The South American teams have been in full flow, so we knew we would be behind when the competition started.
"Our target was to improve every game, which is what we have done. Our displays have shown that."