The calculated distance (air line) between Gelligaer and Southend-on-Sea is approximately 171 miles respectively 275 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 Gelligaer and Southend-on-Sea has been calculated here.
Air line (approximately)
275 kilometers
148 nautical miles