The calculated distance (air line) between Leighton Buzzard and Southend-on-Sea is approximately 64 miles respectively 104 kilometers.
By car or train, the actual journey to Southend-on-Sea is certainly longer, as only the direct route (as the crow flies) between Leighton Buzzard and Southend-on-Sea has been calculated here.
Air line (approximately)
104 kilometers
56 nautical miles