In early episodes, it is obviously the height of summer with people walking around London in t shirts and shorts but, by s1e6 in particular, they can't decide whether it is autumn or winter. In some scenes the trees are turning from green to brown but in the scenes where the released prisoners are followed there are no leaves on the trees at all.
The story is supposed to be based on the duration of an 8 hour flight but looks as though it was filmed over 6 months.