The calculated distance (air line) between Bolton upon Dearne and Southend-on-Sea is approximately 161 miles respectively 259 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 Bolton upon Dearne and Southend-on-Sea has been calculated here.
Air line (approximately)
259 kilometers
140 nautical miles