I didn’t make it to the Kew Royal Botanic Gardens on this trip*, but the British Museum was (and is, til November 25th) holding an exhibit, “North American Landscape: Kew at the British Museum” on its West Lawn. Admission is free.
The website, here, has a video of the installation and a slideshow of the plants featured.
I loved these flower finials atop a rather stately iron gate on Montague Street, not far from the British Museum.
Looking on Google’s satellite image for the street, the gates seem to be the entrance to a large shared garden on the inside of the entire block.