I don't necessarily agree with how my source (see here) has compiled some of the population figures, but at least many seem in line with the data on Wikipedia. It was between citypopulation.de and a somewhat confusing list of urban areas from Wikipedia. Before anyone nitpicks, I am well aware that many of these are not legally cities, but I didn't feel like overcomplicating the title for this one.
I have ideas for more in the series. I may do a few quizzes for regions too, such as Benelux. I'm glad you enjoyed this quiz, I'll keep making them as long as I'm enjoying them.
My source was citypopulation.de and not Wikipedia. I don't agree that Farnborough deserves to be here, although it's all down to the website's methodology.
You've left off cities that other 100k quizzes have: Bradford, Bolton, Rochdale, Rotherham, Poole, Hemel Hempstead, Warrington, Wigan, Oldham, Watford, Stockport, Salford, Sale, etc. I don't understand why.