This is a first upload for me so be nice and enjoy.
How to add the Great Britain super league on FM21Extract the contents of the .zip archive.
Move the .fmf files to:
Documents/Sports Interactive/Football Manager 2021/editor data
Create the last folder if it doesn't exist.
Open FM21 and start a new career save. In the top right of the screen, under "Database", make sure each one of the database is selected.