The term 'Foreign Office' refers to any nation's government department which deals with the rest of the world's countries. They generally issue advice on other countries for visitors, but they also deal directly with other countries' government departments on various international issues.
The Foreign Office was at the time of Crossman (late '60s early '70s - see the Crossman Diaries for details) a virtual anti-Semitic organization. It also preferred to defer to Arabs on everything it could to keep its 'hand in the till' with respect to oil/arms sales (see backlash over Ch4 programme Death of a Princess for further details).
Extremely understanding when it comes to oversees arms sales with respect to Final User Certificates. Alleged to prefer foreigners to the people who pay their salaries.
Considered by some to be nothing more than a room somewhere populated by people with heads full of melted leggo.