I think, from what other people have told me, all GCC Gulf Arab countries go through London Visa Office but I can't be 100 percent sure.
I do not know for sure, but trend for many countries seems to be that some files, likely the simplest applications, are handled in Mississauga, and other files sent to field offices abroad. This may be 'geographic' only in the sense that some countries have a higher proportion of difficult files, e.g. because most/all require more detailed background and security checks.
(I have no knowledge which field offices handle GCC applications, you're probably right()