Coster attended Wadham College in Oxford, where he excelled in languages and modern history, before receiving a commission in the Royal Air Force.

Numerous opportunities arose from this job, one of which was a Commendation for his assistance in recovering the remains of Sir Henry Gurney, the British High Commissioner who was killed in an ambush on Peter's Hill on 6 October 1951.

He completely revolutionised and reorganised the force in this capacity, eventually rising to the position of Head of Intelligence and Security for Sultan Omar Ali Saifuddien III.

[1] In June 1969, a news conference was convened after special agents seized more than 700 items from Chinese propaganda that had been brought in from Hong Kong over the previous two months.

[1] Coster was bestowed the Manteri title of Yang Dimuliakan Pehin Datu Permakawi Diraja by Sultan Omar Ali Saifuddien III on 20 April 1973.