By Muvija M LONDON (Reuters) -The Port of Dover, Britain’s main gateway to Europe, on Friday declared a ‘critical incident’ over long delays, blaming a lack of French customs officials for causing a bottleneck as holidaymakers left for summer breaks. Britain’s departure from the European Union means passengers bound for France face enhanced checks by French officials. The port has increased customs booths to accelerate the process but it said on Friday they were under staffed, leading to gridlock in Dover on what is typically one of the busiest periods of travel, when schools break up for summ…