Gizmo1990 said:
I realise that this is just a dumb action movie but can somebody explain to me why when London is being blown up the only one who can save us is one random American?
Sure, it's not one random American. It's one bodyguard of the man whom the entire terrorist group is focused on trying to capture/kill. So obviously, it's going to give the guy a bit of a lead on the issue when all the bad guys keep coming to him, saves a lot of investigating time.
That said, if the character wasn't around, the plot would have wrapped up around the same time in the same place while being handled by the Brits (although they still have to wait for the NSA to find the bad guys for them, naturally). The only thing the random guy is directly responsible for is saving the President, London sorts out the larger issue itself.