No visa is required for short visits. Route: Standard visitor visa .
The UK government’s visitor guidance confirms that eligible non-visa nationals can visit for up to 6 months using an ETA rather than a visitor visa. The UK government’s current rules distinguish between visa nationals and nationals who can travel with an ETA. The U.S. is not on the UK’s visa-national list.
Processing time: 21–35 days (typical, not guaranteed)