The pilot in command reported that during cruise on a flight from Singapore to London, he began to suffer the increasing effects of food poisoning. Four technical crew members are carried on the Singapore London sector, and when he became ill, the pilot in command transferred control of the flight to a co pilot. After consultation with a crew member who was a qualified nurse, the pilot in command commenced a period of bed rest. About three hours later, his condition improved and he resumed formal control of the aircraft. The pilot in command believed that the source of his illness was food consumed during the stopover in Singapore, and was not the result of a crew sandwich snack, the only food he had eaten in-flight prior to falling ill.