We have walked past this mysterious doorway at least every couple of weeks, and sometimes multiple times a week, and have never known where it goes.  It’s near South Kensington tube station, and is half-way down a high brick wall with no real windows.  There is no obvious signage, and Google Maps has no idea.

This photo taken after dark is not very clear, we will endeavour to revisit in daylight hours but they are few and far between in these winter months.