I visited the new US Embassy in London in February. The email I received about my appointment states:
Location of Embassy, who can attend, and what you can bring: Please see our website at
https://uk.usembassy.gov/u-s-citizen-services/acs-london/, with particular reference to the appointment guide, for information about what you can bring with you to the Embassy, and how to find us at our new location: 33 Nine Elms Lane, London. Please be aware that large bags/suitcases (above cabin baggage size) and laptops cannot be admitted into or stored at the Embassy.
If you bring a laptop or suitcase, they will suggest you go to a cafe across the street that rents storage space. Otherwise, a family member or friend can wait outside with your items.
Phones are allowed inside and can be used.