you can file an application for your spouse to move to the US anytime through the US service centers (if you have a US address)....the difference is it seems to be the rule of thumb that the London Embassy tends to only accept applications for Direct Consular Filing (bypassing the US service centers) when the American spouse has ILR, although it's not a hard-and-fast rule. And also, it's possible they might want your spouse to have ILR as well since he's not a Brit, but I would ask them directly to find out.
In any case, when you eventually move back, your spouse will have to live in the US for three years minimum in order to qualify for naturalization (and there are rules on how much time out of the country is allowed, etc.) You can't do anything to speed it up, but the good news is that if you do DCF with the IR-1 visa (the visa will take a few months to process) your spouse will be a permanent resident upon entry to the States and get a green card valid for 10 years, allowing work immediately along with other benefits. Then, all you do is wait three years and then apply for US citizenship, which will take anywhere from 4 months to a year to process, depending on where you end up living.
edited to add: by the way, it now takes five years, not four, for you and your spouse to qualify for ILR. The rules were changed a few months ago.