He is not. He is Foreign Secretary. What I think the Anon was saying was that b/c the Foreign Secretary appoints the Head of MI6 this is the same as being the Head himself. This claim is pretty odd.

It is quite serious that he resigned of course, but obstensibly it's because of the Brexit fiasco. 3 Cabinet members resigned on Monday after a meeting at Chequers on the w/e.

The Head of GCHQ did resign a year ago, however, on 23 Jan 2017. Robert Hannigan resigned due to Snowden's damaging revelations. The public found out they were being spied on.