I believe it was on the set of 28 Days Later. I have a suspicion it may have been Chris Eccleston's character as those parts were filmed later into the schedul470e (and he'd worked with Chris on Stumpet and Shallow Grave so may have gotten him in at short notice)
The empty London with Cillian Murphy was filmed early September 2001 as the September 11th attacks happened after because Boyle said he'd never have been able to have upturned buses near Buckingham Palace and other trashed things after the 11th.
Don't know who it could have been though.
"What are you, some kind of doomsday machine, boy?"

I dunno but good for him. He's a great guy and a gentleman and I can see him doing this.