I've done a bit of historical research (purely to check the payment systems of our online newspaper archives, you understand) on London's shortest streets.
The upshot of it is as follows - everything is explained in the Google map below, which also has traces of the streets' route with their length. Let me know if you can add anything!
London's shortest named street: Leigh Hunt St, SE1 (36 feet)
London's shortest genuine street: Clennam St, SE1 (85 feet)
London's shortest genuine street until 1909: John-street, SW1 (85 feet)
London's shortest street by repute in 19th c: Mansion House St, EC4 (?c.100 feet)
London's narrowest alley: Emerald Court, WC2 (26.5 inches wide)
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