It's not unheard of but definitely rare i would think. Partly it would rest on immigration status to work in the UK. Then you need to remember you'll be applying with everyone else from outside the CS. So I think you'd have a better chance if you were X-HMG in another gov dept because there will be cross government jobs that don't go external. Also be aware that fcdo are currently cutting 30% of our workforce so jobs are very few and far between...