Hi there!!
Keep in mind that the forms and the instructions are made for people filing FROM the USA, generally. Since you'll be filing by DCF through the Embassy you'll find that the officials at the Embassy who deal with your case are aware that youre living in the UK and that the forms arent really best suited to people in your situation.
So having said that, you need not worry about the "Domicile" issue. The affidavit says you must have a domicile in the USA in order to act as a sponsor for your spouse. Because you live full time in the UK that just isnt possible, and the Embassy knows that.
Now about the assets. You are free to use any or all of your assets based in the UK and in the US, if you have any. Assets include any property you own througout the world, including your home, cars, savings balances, checking account balances, stock portfolios or other investments that can be liquidated within one year. By looking at the affidavit form you'll be able to determine what poverty line requirement you must meet. If you have a household of two it'll be around $15,000 I think. If you wish only to use assets to meet the whole of this requirement you'll need to have assets worth at least 5 times the requirement ($75,000). If you wish to use your UK based income in combination with assets because the income alone does not meet the requirement you'll need to show you have assets worth at least 5 times the difference between the reqirement and your UK income.
Even if youve been working in the UK you'll need to have filed US income tax returns. If you havent done that its easy to file back returns without penalty. You dont have to have all three years worth of requested returns if you didnt have a reason to file, for instance you didnt work during a tax year. If youre not currently employed, thats ok too as long as you have enough assets to meet the requirement. Your UKC spouses' UK based income cannot count towards the income / asset requirement if the income will not continue after you have moved to the US. However if any of the assets you wish to use are in your spouses' name only, you are free to use those as if they were your own.
So no need to panic. All of your assets in the US and UK are fine to use on the affidavit.
Hope that helps.
Sarah
PS: You might find this helpfull. I posted it a while ago with quite an extensive and longwinded section on the affidavit lol
http://www.talk.uk-yankee.com/index.php?topic=3987.0