Hamilton and team-mate Jenson Button had been hopeful of a strong showing on home soil, but the pair struggled for pace and could ultimately manage only eighth and 10th place finishes respectively.
Red Bull's Mark Webber took his ninth career victory with Fernando Alonso and Sebastian Vettel rounding out the podium, leaving Hamilton 37 points adrift of the Ferrari driver in the title chase.
Button is 79 points behind in eighth place, while McLaren also dropped to fourth in the constructors' championship behind Lotus.
Having been outpaced by Red Bull and Ferrari at two contrasting circuits in Valencia and Silverstone, the team appear to have a sizeable task on their hands to catch up to their rivals.
And Hamilton said: "We are still in the fight but unless we find something it is going to be hard to stay in the fight.
"I raced my heart out as always but we just struggled, we did not have enough speed in general.
"I wish we could have done better at my home grand prix but the others have clearly made another step forward so it was a real tough race.
"We have a lot of time to find from somewhere, we need a lot of downforce from somewhere.
"I don't know where the guys can find that but I really pray that they can find it."
McLaren have little time to find the required improvements to the MP4-27 with just two weeks until the German Grand Prix at Hockenheim.
Watch: Webber wins British Grand Prix.. More Videos
Hamilton does not expect an upturn in form to arrive in time for that race, one of three taking place over four weekends during a crucial phase of the season.
"I am sure we will have a very similar car (at Hockenheim) but with some upgrades," he said.
"I hope we can have a better race. These guys in front have really picked up.
"Mercedes, Lotus, Red Bull and Ferrari have all picked up a lot of pace so we are a little bit behind at the moment.
"I expected us to get at least fifth place but those four teams in front of us were much quicker than us.
"The fans were incredible this weekend and it's a shame we couldn't give them a better result."
Button was similarly bleak in his assessment of the team's speed.
He said: "The cars I was racing with, the Saubers and Williams, I was surprised I was racing with them because they must have had bad strategies or something as they were much much quicker than me.
"It was a disappointing day and as a team we have a lot of work to do."