Put this in your vimrc:
let @a='ICo-authored-by: �ly$A <@digital.cabinet-office.gov.uk>�F@Pva<guf r.'
Then you can expand a name into a co-authored line by pressing @a in normal mode, like so:
In:
Richard Towers
Out:
Co-authored-by: Richard Towers <richard.towers@digital.cabinet-office.gov.uk>