THE Guildhall String Orchestra, one of England's most brilliant young ensembles, makes its debut in Hongkong on April 23 at 8 pm at the City Hall Concert Hall. The Guildhall String Orchestra has carved out an impressive artistic reputation as one of just two string orchestras without a conductor in England. Formed in 1981, the orchestra comprises 11 string players plus harpsichordist. All the musicians studied together at the famed Guildhall School of Music in London. Based in London, the Guildhall String Orchestra regularly appears at major British festivals and frequently broadcasts on BBC radio and TV. Their engagements abroad have included performances in Spain, France, Italy, Germany, Denmark, Austria, Switzerland, Greece, Yugoslavia, Hungary, Belgium, Luxembourg, Holland and Algeria, with television broadcasts in Canada, the United States and Europe. The orchestra has also won several major awards and prizes. Programmes of the orchestra's concert here include Bartok's Romanian Dances, Holst's St Paul's Suite, Mozart's Adagio and Fugue, Grieg's Holberg Suite, Elgar's Elegy for Strings, and Tchaikovsky's String Serenade in C. Tickets at $160, $120, $80 and $40, with half-price tickets for students and senior citizens, are available at all URBTIX outlets. For telephone reservations call 734-9009.