This defeat by Australia in Melbourne yesterday, following their opening 24-10 loss to New Zealand, means the tourists will fight it out with Papua New Guinea in Auckland next Saturday to avoid the wooden spoon.
England briefly threatened to spring an upset when they led
8-6 after 14 minutes thanks to a try from Sam Burgess and two goals from Ben Westwood.
But they let the opportunity slip in a crucial 10-minute spell in which they conceded three tries after a series of elementary handling lapses to trail 26-8 at half-time.
"We made too many errors, play-one errors, it was as simple as that," said McNamara.
"Physically, I thought we were very dominant at the start of the game, both offensively and defensively, but we hurt ourselves with some errors and Australia seized on those opportunities."
England were out of luck with a couple of decisions from video referee Steve Clark, who failed to spot a knock-on in the build-up to Luke Lewis's second try, while Lote Tuqiri's second-half score followed a blatant forward pass.
McNamara, who made wholesale changes including giving debuts to Leroy Cudjoe, Shaun Lunt and Ben Harrison, declined to use the poor decisions as an excuse, however, as he looked on the bright side.
"There were some very contentious calls," he said. "The players clearly saw it as a knock-on when the second rower scored but I'm not here to whinge and moan about that.
"If we hadn't made the errors that gave the field position, we wouldn't have had to worry about some dubious decisions.
"There was a period in the game where we came up a load of early-play errors but, other than that, there was not a great deal to choose between the teams.
"There is frustration from our players because some of them know they can perform a little bit better but this team has an average age of 24, a captain who is only 25 and another starting prop who is 21.
"There are some outside backs who probably haven't got 10 caps between them.
"We came here with the sole intention of winning this competition but also making us stronger for the future of international rugby league and I certainly think we've made some strides forward.
"The players gained some experience (against Australia) which will certainly make them a whole lot better."
Captain James Graham admitted his side once more paid a heavy price for their inability to compete for the whole 80 minutes.
"We're bitterly disappointed," he said. "We feel we can perform a lot better. We showed that in glimpses but it's not good enough. We need to do it for 80 minutes, not 10 minutes here and 10 minutes there."
Australia coach Tim Sheens had some sympathy for England, who were without a host of players through injury and unavailability.
He singled out sidelined captain Adrian Morley and
St Helens scrum-half Kyle Eastwood as particularly severe losses for the tourists.
"It's difficult for England," he said. "They're out here with nowhere near their best squad and England have to have their best squad available.
"They missed the young fella from St Helens and losing Morley early and having a number of other players unavailable does not help England."
Sheens was highly critical of the performance of Australian referee Tony Archer, accusing him of allowing England to lie on in the tackle, a claim his opposite number was quick to refute.
"I have to say I was a bit disappointed with the speed of the game," said Sheens. "It was a joke, to be honest. They choked us on the ground.
"I have to say that standard is not acceptable. There was a different interpretation of the speed of the play-the-ball. It was obviously designed to slow us down."
McNamara countered: "The speed of the ruck was terribly slow at times and it damaged us more than it damaged them because we were pushing through the middle and they were struggling to hold onto us with Burgess, Westwood and Graham particularly strong."
Meanwhile, McNamara said he would have no difficulty motivating his players for their wooden-spoon duel with the Kumuls at Eden Park.
"The players are bitterly disappointed now but we'll attack the game next week with great enthusiasm and finish on a high," he said. "We need to put an 80-minute performance together.
"We've played two extremely good sides and we've been a little bit off our best in both those games. We will learn hugely from it but we want to finish off very well next week."