Starting January 8, 2025, U.S. citizens traveling or transiting throught United Kingdom will need an Electronic Travel Authorization.  This requirement applies to all parts of the UK, including Northern Ireland and British Overseas Territories.

          Easist way the apply for The Electronic Travel Authorization (ETA)  on your iPhone or Android phones. The App names UK ETA. You can see more details on this link. 

         Application processing time usually takes within 3 working days, but you may get a quicker decision. If you need travel to UK in urgent and have not time to wait ETA, just need the apply ETA before the flights and you can travel the UK while waiting the ETA approval. Application fee only £10, around $13 and application takes around 10 minutes. 

         You can check the transit visa eligibility on this link  if you ve have US permanent residence (Greencard) or have USA visa.