The MAIL calls this a shelf cloud.
It’s a Cumulonimbus capillatus.
Note to the MAIL: You have one of the World’s top experts on clouds. Pick up a copy of the CLOUD BOOK by Richard Hamblyn. It’s published by the MET OFFICE.
Oh, and here is a shelf cloud:
All you had to do was ask somebody who knew something about clouds…
The soap box is now free….