Revealing mistake: When Jim is walking across the Westminster bridge and shouts "Hello?" for the first time, in the next shot you can see a blob on the bridge (right of picture). This is all that's left of a car going across the bridge, that wasn't painted out completely.
Factual error: In the scenes driving to Manchester the motorway is completely clear of traffic, no abandoned vehicles of any sort. With an outbreak like this happening the motorway would be gridlocked.