Why moving to the UK does not end US filing
The United States taxes on the basis of citizenship, not residence. The IRS confirms that citizens and green-card holders abroad remain subject to US tax on worldwide income from all sources, so a US person living in London files a Form 1040 just as they would in New York. Americans abroad get an automatic two-month filing extension to 15 June, and reliefs such as the foreign tax credit and the Foreign Earned Income Exclusion are only available if you actually file. Our full guide, Americans living in the UK: tax explained, covers the whole system in detail.
The UK return sits alongside it
Most Americans who move here become UK tax resident under the Statutory Residence Test, and GOV.UK is clear that UK residents normally pay UK tax on their foreign income as well as their UK income. That usually means a UK Self Assessment return each year on top of the 1040. Filing twice rarely means paying twice: under the treaty and the foreign tax credit rules, the country that taxes second gives credit for tax already paid, and because UK rates are generally higher, the UK tax often covers the US liability on the same income. The treaty saving clause still lets the US tax its own citizens largely as if the treaty did not exist, which is why UK products such as ISAs are not US tax-free.
Do not forget the account reports
On top of both returns, FBAR (FinCEN Form 114) is due if your non-US accounts exceed $10,000 in aggregate at any point in the calendar year, and FATCA Form 8938 applies at higher thresholds; see FBAR and FATCA explained. Horizon, a UK Chartered Tax Adviser practice, prepares the UK side on fixed fees agreed upfront, with US filings handled by our US partners (Enrolled Agents and CPAs), whom we coordinate for you.