I once found a instagram video from a guy who helped ITV film them. I have no idea if the video still exists but in the video it was a typical cloudy, rainy day. He said they later had them professionally edited by Coffee & TV with the program Smoke.
From Coffee & TV:
“We needed to deliver 2 versions – a 6am version which reflected the warm glow of sunrise across the city and an 8am version which was designed to portray a much clearer blue sky.” said Truss, “This was quite a challenge as we were using the same footage for both versions with 2 shots that had been filmed on different days, but both typically grey London days so I had to use atmospheric lighting and sky replacements in order to achieve the desired result.”
You can see the project details here: http://www.coffeeand.tv/work/new-title-elements/