AMONG those in the British Chamber of Commerce trade mission in Hong Kong this week are Frances and Andrew Dumbleton, who have an unusual link with the territory. The Dumbletons run a family photographic studio in Cambridge, and since it was established 20 years ago it has been the studio of choice for the graduation portraits of Hong Kong students at Cambridge University. Said Frances: ''The Hong Kong students are always so delightful, well-groomed and polite - and they take an excellent picture.'' The Dumbletons had a pleasant surprise early this week when Bank of East Asia chairman David Li - who met them at Cambridge, where his two sons are at university - sent a huge bouquet of flowers to their hotel. And that's not all. The banker has commissioned them to do the official portraits when the bank celebrates its 75th anniversary next year. During their visit they have been making arrangements for a longer trip to the territory at the end of December and they look forward to linking up with former Cambridge clients. ''We are looking forward to coming back, since Hong Kong is such an easy place to do business. Everybody is nice and easy to deal with and so professional,'' Frances said. Anyone wanting to contact the Dumbletons when they are here in December can call Melanie Elliott on 605 6654.