If you are a US Citizen (or a Green Card Holder) and that you are living in the United Kingdom, you may be required to still prepare a US tax return with the IRS. This is because America taxes based on citizenship and not residence. In addition, you will be required to declare worldwide income on Form 1040.
Depending on the level of tax that you may have to pay to the IRS, you could be have to prepare returns for a number of years if you have never prepared a return since leaving the USA.
If you are a US citizen living in the UK and would like to discuss this, please do not hesitate to contact us at firstname.lastname@example.org. We look forward to hearing from you.