As of January 8, 2025 the United Kingdom will require US passport holders to secure an Electronic Travel Authorization (ETA) if visiting or transiting through the UK, even if you are not going through UK border control before connecting to another flight. Travelers may apply online for the ETA using this link:

Apply for an electronic travel authorisation (ETA) - GOV.UK

The ETA is typically processed within 3 business days, costs approximately $12 and is valid for two years from the date of issuance.