Wayne Bennett's side were too strong for France as they recorded a 36-6 victory on Sunday, but their second-half performance was below par once again.
Tries from Gareth Widdop, Stefan Ratchford, James Graham, Mark Percival, John Bateman and Jermaine McGillvary ensured England booked their place in the last eight.
They will take on Papua New Guinea in Melbourne on Sunday but sceptics are still expecting England to struggle in the final few matches of the competition.
While Betts was pleased with the start, he admits everyone in the camp know they need to improve for next week.
He told BBC Sport: "It was a great first half, just a very disjointed, sloppy second half.
"I was happy we did not concede another try, keeping them to six, but the completion rate was not good enough, ball control was really poor and the decision-making and options were not good enough.
"It is hard to put a finger on why. I have got to highlight how clinical we were at the start and some of the attacking skills were outstanding but we have got to do that for 80 minutes."