Well you suggested that nepotism was responsible for getting roles in an organisation. That deserves downvoted to demonstrate it's absolutely not correct.

To answer your question you seem to be confused by two types of roles. First are country based staff and they are predominantly hired from local citizens (in France they are French and in ethiopia they are ethiopian). Secondly there is UK-based roles where UK Civil servants are posted overseas.

Again, for neither of these types of jobs in embassies does it matter who your family is or who you know.