Shaun of the Dead/WMG

The Zombification process accelerates as the disease develops.
Shaun's stepfather needs hours to turn into a Zombie; Shaun's mother, only an hour or so; Ed, only several minutes.
 * No, it can be explained by Max Brook's Hypothesis; there are different rates of "burn" which is regulated by where you are bitten. We never see where Phillip is originally bitten, but after he is bitten in the shoulder he only lasts (+ -) as long as Ed whereas Shaun's Mother is bitten in the wrist so she can last a while, but not to long.
 * Phillip's comment of "I ran it under a cold tap" and his sling would imply that the bite was on his hand or arm.
 * It's possible that the rate of zombification is different for different people.
 * Yeah. Maybe it's based on metabolism or blood type.
 * In addition to the above, while Barbara and Phillip got relatively mild bites (at least the first time around in Phillip's case), Ed got part of his neck bitten out, and Phillip's condition worsened significantly after his second, more savage biting; both Phillip and Ed changed quicker because they were bleeding to death, so they died sooner and reanimated quicker.

Shaun's original plan would have worked
We take Pete's car, we drive over to Mum's, we go in, take care of Philip, then we grab Mum, we go over to Liz's place, hole up, have a cup of tea and wait for this whole thing to blow over. This would have worked- Shaun's Mum wouldn't have been bitten, Liz's place had security doors, was off the ground and had supplies, and the whole thing did blow over in a fairly short time. If only Ed hadn't moaned about it, they would have been OK. And if he'd known that Liz had taken up smoking again, he could have been persuaded. It just goes to show that Shaun's hidden talent is as a zombie survivor.

Shaun of the Dead takes place in the Dawn Of The Dead universe
England survived the zombie apocalypse but America and the other countries fell. At the end, England is the last country standing.
 * I pity England, then. With the sheer amount of economic cross-dependence necessary to keep the modern first world working, the complete collapse of every other nation would hit the place pretty hard. Expect third world country conditions, at best.
 * In addition, V for Vendetta takes place in the same universe. All of the world but England has collapsed, and England has become a dystopian place ruled by a dictator.
 * Seconded. Also, the Zombie illness is the Saint Mary's disease.

Shaun is immune to the zombie virus.
This is just a possibility, but considering he had his mouth wide open while inflicting a can of cricket-bat whoopass on Mary (and in the immidiate following scene is shown with splatters of red on his torso and the lower part of his face), it's not too far of a stretch to imagine a drop or two of infected blood could have flown in his mouth. Go and watch the movie again with this concept in mind.
 * Healthy human saliva defeats the virus! Oh if only more people had just licked the zombie bites!
 * Or everyone has an immune system and can counteract minor contact with the virus. If there was even a virus. It's never explained.

Running a Zombie bite under a cold tap cures the infectee.
Phillip was bitten long before Shaun and Ed got to his house. He shows no noticeable difference from his normal(Admittedly, already zombie like) disposition, and his hand isn't bleeding the way the other zombie bites are. He doesn't even seem phased until he gets bitten a second time. Clearly, this whole outbreak could have been averted if more infected ran their injuries under a cold tap.
 * That's actually perfectly reasonable. It's warm weather in the northern hemisphere when Shaun of the Dead takes place, and as far as I can recall, nobody else immediately iced their wounds. It's certainly reasonable that whatever causes zombification (a protozoan? A slime mold that breeds primarily in liquids that contain similar enzymes to those in saliva? A curse designed by the original inhabitants of Britain to combat the Romans, who would be too weak to survive the winter if they attempted to follow the northerly rearward advancement of their target populace but needed to be weakened and confused?) dies when it reaches temperatures of less than, say, sixty-eight degrees. Phillip running near-freezing water over the bite before the cause got too far from the surface killed some and weakened any possible survivors enough that his immune system could destroy them.

This is the same universe as Left 4 Dead
And it turns out that Britain is just plain better at containing the infection than America.
 * Well if American Infected moved as slowly as British Infected, then we wouldn't have had such a problem. By the way, how did you guys do that? What prerequisite was necessary to make your zombies re-animate into crippled retards? Do you think it was the coffee/tea difference? Both have caffeine, but I think coffee has a bit more.
 * Left 4 Dead's zombies are Technically Living Zombies, so they have better cognitive ability and don't rot or decay. It's heavily implied that Shaun of the Dead's zombies are actually proper, real dead men walking, so they're brain dead and don't have particularly good muscle strength. Ta-da.
 * Green Flu in America was a mutated version of the plague in England. While the England plague focused on killing the person's cognitive functions while boosting resistance with a little to no incubation time, Green Flu focused on incubation and swapping out some of the energy dealt by the brain and using it for stamina and further mutation. Thus, England has a disease that kills lots in it's first incarnation but is easily dealt with otherwise, and America has an insidious disease that can take weeks to make you into an undead if you are unlucky and has mutated infected that can think. Shaun of the Dead would be very different if both diseases were the exact same.

Phillip wasn't actually infected.
At least, not until he was attacked next to the car. He seems perfectly normal up until the second bite, and reanimates fairly quickly, just like Barbara and Ed do later. Maybe he was just attacked by some druggie or crazy person and would have been fine if it wasn't for the zombies outside.