Hiya Carla,
to answer your original question - you can indeed apply for an immigrant visa while resident in the UK. An immigrant visa is basically your husband's ticket through the doors of the US. When he gets to the US with this visa in hand, they'll stamp his passport with a temp "green card" until his actual green card arrives, which takes a couple of weeks. In a previous post a few hairs were split about the immigrant visa / green card thing. The end result of all this application business is indeed a green card.
Anyhow... if youre resident in the Uk you can apply for a petition to take your husband with you back to the US through the US Embassy in London. This process is called Direct Counsular Filing (DCF). You can do all of the applications while in the UK and you can do this any time you like.
You did say that you didnt have any real intention of immigrating at the moment but that you wanted the option there and ready to go should you suddenly fancy a move. Thats not really possible. You can NOT maintain both a green card and a primary residence in the UK. Doing so will result in the green card being revoked. You can reapply, but they would likely take into consideration on the second application that the first green card was revoked and why. Keep in mind a green card is a RESIDENCE permit, not a passport.
Once you have the immigrant visa you have 6 months to use it. Once you have the green card stamp in your passport you can remain out of the US for up to one year at a time. After a long period of being out of the US you will likely have to show why you spent such a long time abroad. A valid reason would be liquidating one's property to facilitate the move to the US. The big rule that you should keep in mind is that once you have the green card you HAVE to spend the majority of your time LIVING in the US in order to maintain the green card. You cant live in the UK and keep that green card in your back pocket for a rainy day.
But really once you DO decide to make that move, it doesnt take very long to make the applications. Generally the people here have had their visas within a couple months. So having the facility already prepared isnt a problem. You'll have it all sorted before you know it
Best of luck!
Sarah
PS: Hope that makes sense. If you have more questions about time limits and being abraod while having a green card, just lemme know. Martin and I are experiencing this issue first hand as Martin already has his green card but we are stuck in the UK trying to sell up. Personally I have to advise strongly against applying before youre 100% ready to go. We've not been in the US for 6 months and every day we're out of the US the higher the risk that martin could loose his green card. Its not a very nice position to be in.