Good morning

I have a long layover at LHR and wanted to step outside and visit London. Any idea if this still allowed? I have a US passport so no visa issues. I know there is a new electronic travel auth that will be needed.

Flight details.
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Delta 5944 10:40am • Mumbai, India (BOM-Chhatrapati Shivaji Intl.) to London, England, UK (LHR-Heathrow) Terminal 2 9h 40m flight duration Delta, operated by VIRGIN ATLANTIC

Layover: 20h 55m

Delta 5983 12:45pm • London, England, UK (LHR-Heathrow) to Washington, DC, United States (IAD-Washington Dulles Intl.) Terminal 3
Economy / Coach (T) 8h 25m flight duration
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Many thanks in advance.