ONE of London's best-known theatres is looking for a Hong Kong buyer. The Playhouse was owned by novelist and former Conservative Party deputy chairman Lord Archer until he sold it to producer and author Ray Cooney two years ago. Soon after he bought the West End theatre, Cooney admitted he was ''pretty silly with money'' and had paid more than he should have. It is now on sale for GBP2.8 million (HK$32.34 million). He said then: ''It cost GBP2.5 million to buy and restore . . . but theatres are like fine paintings - very rare.'' Now Cooney, who wrote Run for your Wife, has advertised the theatre in the South China Morning Post. Asked why he chose Hong Kong, he said: ''It struck me that a lot of my plays have shown there and also that there would be a lot of finance there.''