RELATIVES of Hong Kong people jailed in Thailand yesterday took their case to the Legislative Council. They want the Hong Kong Government to ask Thai authorities to improve conditions and clarify details of a prisoner exchange programme. Wong Siu-mei, whose brother is serving a 50-year term, said the family had to pay a great deal to get his brother a ''decent'' life in jail. ''Without paying a bribe, he will have to live on muddy water and coarse rice served by the guards,'' she said. The family is considering asking for Mr Wong to return to Hong Kong to complete his jail term.