There have been lots and lots of questions about whether or not to file a tax return, with lots of people on the outside of the forum giving lots of bad advice. This thread should help answer that question.
Generally speaking, if you are a US Citizen and filed tax returns before leaving the States, you must continue to file a tax return, even when you are living abroad. Though, generally, if you do not earn in excess the equivilant of 80,000 USD you will not owe any taxes. To actually figure out if you must file is rather complicated and you should check out the table at: http://london.usembassy.org.uk/irs/irswho.htm
So you just realized you haven't filed but you were supposed to and are worried you are in trouble. The official answer is:
This is a common misunderstanding among Americans abroad, and should not create anxiety for those who find themselves in this situation. Generally, you should file returns for the past three years, taking the foreign earned income exclusion, the foreign tax credit, or both. It would be extremely unlikely that there would be any late penalties assessed, since penalties are computed as a percentage of tax owed, and only the rare taxpayer would actually owe tax in this situation.
You may contact us for prior year tax forms, or download them from the IRS web site. To find older forms, enter the year required in the Search for... box on the top-left of the page. The search typically returns Forms and Publications for the year on which you will have seached.
and can be found here: http://london.usembassy.org.uk/irs/irsfaq.htm
If you were a green card holder (resident alien) of the US, you may also need to continue to file tax returns for a period of time.
In all cases, it is always good to seek out good tax advice from a professional, which is often a tough question. The UK Tax Accountants can be a rather complex subject. Stealing from a post in another section here:
The UK has several qualifications - an adviser can be a Chartered Tax Adviser (CTA) by passing a series of exams and holding practical experience (the Which? report calls this an ATII qualification, but the name changed 3 years ago to CTA). An adviser can also be a Fellow of the Chartered Institute of Taxation (FCTA - awarded by a body of work); a member of the Association on Tax Technicians (ATT) by passing a series of exams and holding practical experience; a member of the Federation of Tax Advisers (FTA - again by passing a series of exams and holding practical experience); a Fellow of the Federation of Tax Advisers (FFTA - awarded to the more senior members) and a member of the Tax Faculty of the Institute of Chartered Accountants in England & Wales.
The problem is there is no professional organization that monitors or reports on accountants that are certified in both US and UK taxes, which is then complicated more by the situation that even if someone knows both US and UK taxes they may not have a good understanding of how they interact. In complex cases (lots of money), you would really need a firm that has specialists in several areas.