With four differences to go through, we won't waste time going through these answers. First up, did anyone notice the route number on the bus to the right? In the first image it's the 148 and in the second one it has changed to the 203. Just above that bus, there's a cloud in the first image. When you look to the second image you can see it has disappeared.
Big Ben's pillars are looking magnificent in the first picture, but by the second picture, the furthest one to the left has somehow gone missing. Finally, let's reveal the most difficult. Anyone with a keen eye will have noticed that the time is different on each clock face. In the first image it's quarter to ten and in the second image it's ten past twelve.