silly language question

[Bosje]

just a quick one: I know it's 'he lives south of london'
but I always thought that for nautical descriptions you would say:
'the ship sailed just south off Rockall'

but now i'm in doubt, lol
is that correct or am I mixing something up, is it simply 'just south of Rockall' as well?

thanks

edit: can't resist adding this as an unrelated bit of fun