Are they?

   London - 8.8m - https://en.wikipedia.org/wiki/London
   Birmingham - 1.1m - https://en.wikipedia.org/wiki/Birmingham
   Manchester - 552k (2.7m urban region) - https://en.wikipedia.org/wiki/Manchester
   Glasgow - 632k (1m in the urban region) - https://en.wikipedia.org/wiki/Glasgow

They're not strictly accurate, but are reasonably close. Also, those Wikipedia numbers are from 2021.