I'd have the humans in the game replaced by plants, and turn it into a film about those plants beating the zombies in a tower-defense situation, kind of like a game, but it's a film instead! I'd call it "Plants Against Zombies"...
Okay, seriously though, I'd probably just make the zombies the good guys as well. Or I'd set it in the UK and have absolutely zero mention of the USA at all. That's fairly unique for a zombie film, the only one I know of that's UK based without any US intervention of any kind is Shaun of the Dead. 28 Days Later has the USA at the end, and Dead Set doesn't count because that was a TV miniseries, rather than a single feature-length film. Though I did love it, and Charlie Brooker is bloody awesome.
Yeah, I think that'd work. Have it set in the UK, have the population of the UK, along with the government, evacuated to France and western Europe, and have the UK government being granted permission to set up a series of refugee facilities and camps and a new power-base to maintain order in said camps along the northern French coast, and the film could be based around UK troops and missions into the restricted zone (i.e. the main British island) to search for survivors, find evidence to how the outbreak began, and other things. The main focus would be a small team of MI5 operatives who have been given special training and are sent into the restricted zone on an operation to find information on the Umbrella-esque company responsible for the outbreak, and to try and deal with that threat by finding the company heads and bringing them to justice before there are any further outbreaks that aren't as easy to contain by way of them not being on an island like Britain is...