The garden is located in the center of St James's Square and it is frequently used by the locals as an exhibition venue. For this reason, the garden is not open to the public freely, it is reserved for the residents and freeholders.
The gardens are usually open to the public on weekdays from 7:30 am to 4:30 pm. They are kept locked and accessible only to freeholders and residents at other times.
Wikipedia https://en.wikipedia.org/wiki/St_James's_Square
Address London, SW1, England
Coordinates 51°30'25.816" N -0°8'6.928" E