Simson began to compete in shot put as a teenager at Thurrock Harriers in Grays, Essex. He won the junior (under-15s) category at the 1984 English Schools Championships, was the intermediate (under-17s) champion in 1986, then succeeded Simon Williams to the under-19s English Schools' title in 1987. He established himself as the top young shot putter of his generation, winning four straight titles at the AAA Junior Championships and three straight AAA Junior Indoor titles. He set national indoor age bests for 16-year-olds in 1987 and 17-year-olds in 1988. In international competition he placed seventh at the 1987 European Athletics Junior Championships before improving to take the bronze medal at the 1989 edition. He was fifth in the shot put at the 1988 World Junior Championships in Athletics.