I think Boris to the FO is rather a shrewd move, TBH. It has an element of payback - "you created this international mess, now you help clear it up".
Also, as one Guardian commentator has quipped;
Johnson’s appointment represents a glittering promotion for him - but perhaps a demotion for the Foreign Office. Superficially it is an odd appointment, because it won’t take long to compile quite a long list of countries and foreign leaders Johnson has offended in the past, but he likes to be liked and he may forge good relations with his counterparts. But the new arrangements also suggest the Foreign Office is being downsized. It used to be in charge of aid, but under Labour it lost that to DfID. Now it is losing Brexit (to Davis’s department) and international trade (to Fox’s department). What’s left for Johnson? Handing out the Ferrero Rocher? http://www.theguardian.com/politics/blo ... itics-live (21:15)
And as another commentator has said, what other high profile job can you give someone where if they make a total hash of it, it'll only ruin their own career, not the Government?
Finally, I bet May lurrrrved telling George, "You want the Foreign Office? Nah, we've already got a trained monkey for that. Now on yer bike, matey ...."