That might be part of the Rage Virus Zombie Mark II that is a smarter, faster zombie (if that is indeed what they were intending, rather than just a massive loss of continuity). As I said earlier, though, why wouldn’t the Chunnel have been shut down the second people realized what was going on in England?
They also had an awful lot of infected running around in broad daylight. Didn’t the first movie set it up so that they really didn’t like the light? That’s why you were relatively safe during the day as long as you stayed out of dark places.
I thought the mom was mostly just a little feral because she was traumatized. When the boy got infected, he wasn’t the slightest bit grumpy or bitey. They were immune carriers.
Didn’t Carlysle state that he was given some sort of top-notch pass that would give him access to everywhere in the complex?
If so… why?
A couple people have mentioned the Chunnel, but I don’t recall it featuring in the movie. Maybe I’m misremembering. The end, though, was not about zombies going through the chunnel; it was about the little boy being a carrier. He was airlifted to the continent, and sooner or later, he scraped his knee or shared a soda with someone, and all hell broke loose.
I also think that a serious consideration of containment protocols would not have included an emergency evacuation procedure. Anyone who goes back to England stays there for the duration, or at least until several years have gone by without consequence. Another outbreak after six months? There’s no way they’d evacuate under fire and risk bringing it to the mainland.
In looking for confirmation that zombies came up from the Chunnel (it looks like it - we’re far from the only ones who think that is how the movie ended), I ran across another point of contention - the head of the medical team in England is way too young for this kind of position. This would be the most important medical position in the entire world; it goes to the person who has the most experience because the stakes are far too high, and at her age, it just wouldn’t be her.
According to Wikipedia, this is just a regular tunnel and the virus most likely spread because of the kid.
Just a tunnel, not THE chunnel? I guess I can live with that. 
I don’t recall anything from the first movie suggesting that they didn’t like daylight. Jim probably didn’t want to go into the underground tunnel because there was likely to be a pileup (which there was) and because they wouldn’t know how many infected were down there. Selena says “don’t go anywhere at night, unless you’ve got no choice,” but I think that was mainly because the infected would be harder to detect than in the day. They may have been more numerous at night, but I still don’t remember them having any beef with daylight.
That may have been an homage. It was arguably both the final death in 28 Days Later and the first death in 28 Weeks Later. That is assuming that you don’t count Major West at the end of Days, nor any of the cottage dwellers at the beginning of Weeks. I suppose you’d also have to discount the infected killed during the defense of the cottage in Weeks.
It was most likely one of the kids.
Dad wasn’t immune - Mom was. So one or both or neither of the the two kids may have been immune. The endgame involved the kids and the surviving pilot helicoptering across the Channel into France.
Never mind the plot hole where non-cleared flights from England aren’t identified/shot down/quarantined…
So one or more of the kids and the pilot are infected, and the infection takes hold in France, with the tunnel scene being part of Paris falling to the virus.