New York City is about 312 times smaller than United Kingdom.

United Kingdom is approximately 243,610 sq km, while New York City (including all five boroughs: Manhattan, Brooklyn, Queens, The Bronx, and Staten Island) is approximately 781 sq km, making New York City 0.32% the size of United Kingdom.
Tip: This to-scale comparison of United Kingdom vs. New York City uses the Mercator projection, which distorts the size of regions near the poles.