You are a USC living in the UK, so for your husband to get a US green card, you use DCF with the American Consulate in London. You are allowed to do this because you are a resident of the UK, albeit a Limited resident (no ILR), but a resident none the less. I guess you do want to be careful, though, because I think it is averaging 4-6 months right now using DCF, and if you are right at the 2 year mark, your current visa may expire, and you don't want that to happen!