London is about 137 times smaller than Guyana.

Guyana is approximately 214,969 sq km, while London (including the 32 boroughs of greater London) is approximately 1,572 sq km, making London 0.73% the size of Guyana.
Tip: This to-scale comparison of Guyana vs. London uses the Mercator projection, which distorts the size of regions near the poles.