Effective January 8, 2025, all U.S. citizens transiting through or traveling to the United Kingdom for tourism, family visits, business meetings, conferences, or short-term study for 6 months or less will require an Electronic Travel Authorization (ETA) prior to travel. ETA will valid for 2yrs.  Information on the new requirements and how to apply can be found on the UK Government website
 
US citizens can apply through App by Installing “UK ETA” from App Store or on Google Play or apply online https://apply-for-an-eta.homeoffice.gov.uk/apply/electronic-travel-authorisation/how-to-apply 
Requirements:
  1. Take or upload a photo of the passport you will use to travel to the UK.
  2. Scan your face with your device, if it has a camera. Children aged 9 and under will not be asked to scan their face.
  3. Take or upload a photo of yourself.
  4. Answer some questions about yourself.
  5. Pay for your application.
  6. Cost is 10 Pounds per person.
  7. ETA processes within 3-5 business days. Good for 2 years or expiration date of Passport if less than two years.