You could say that about any team, but it's not how cricket works. Hell, you could say Australia aren't the best team in the world until they beat England, India, Pakistan and South Africa again. After all, it's been more than two years since they last played India in a test match, and they haven't faced Pakistan since Asif turned the corner, blah blah. It's just silly. Until England actually get beaten by somebody in convincing fashion they're the second best team in the world, because they've earned that place with consistent results in test cricket. In the last two seasons they've beaten South Africa away, Australia at home, Pakistan at home and drawn India away. The only real blemishes are an away loss in a fairly tight series to Pakistan and the home draw against Sri Lanka, along with the hammering they just copped in the Ashes.
Losing 5-0 to Australia doesn't indicate they aren't the second best side any more than losing 5-1 in 01/02 indicated South Africa wasn't the second best side. Of course, if England fail to beat the West Indies and India at home they'll lose the "clear number two" tag and it'll be up for grabs again, but for now it's pretty clear cut.