http://www.irs.gov/publications/p54/index.htmlFiling Requirements
If you are a U.S. citizen or resident alien, the rules for filing income, estate, and gift tax returns and for paying estimated tax are generally the same whether you are in the United States or abroad.
Your income, filing status, and age generally determine whether you must file an income tax return. Generally, you must file a return for 2004 if your gross income from worldwide sources is at least the amount shown for your filing status in the following table.
Filing Status* Amount
Single $7,950
65 or older $9,150
Head of household $10,250
65 or older $11,450
Qualifying widow(er) $12,800
65 or older $13,750
Married filing jointly $15,900
Not living with spouse at end of year $3,100
One spouse 65 or older $16,850
Both spouses 65 or older $17,800
Married filing separately $3,100
*If you are the dependent of another taxpayer, see the instructions for Form 1040 for more information on whether you must file a return.
Unless the IRS wants to come after you about not filing, you shouldn't have a problem especially if you can prove you did not earn money in that tax year. Any money made under $80,000 in the UK is exempted from US tax through a tax treaty.