essjay -- it is super, super-easy for us DCF folk. We have very little to prove beyond re-establishing domicile (and we all know London is very lenient too, so a non-starter!) and brandishing an I-864 or two. You are going to walk out of the Embassy doing either this:

(wtf? that was so easy, I was an idiot to worry!) or this:

(wtf? that was so easy, I was an idiot to worry and now I must dance on the Embassy steps!).
Okay, okay, I did the latter. You may be possessed of more decorum and wait to do the happy dance until later!