The calculated distance (air line) between Irthlingborough and Southend-on-Sea is approximately 78 miles respectively 126 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 Irthlingborough and Southend-on-Sea has been calculated here.
Air line (approximately)
126 kilometers
68 nautical miles