Many thanks @guv1976 for the help in advance. Many thanks also for the great suggestion but I normally search flights on skyscanner, play around until I get the right combination. Skyscanner do not normally have very long stopover and it seems more complicated then if I have to find an agent, as I would lose the interactivity of playing around to find the right combination.

Furthermore, it is also a bit more complicated. The journey I described is for our daughter for her 'short' two-week holiday in HKG, but we want to put a 'seamless' trip to Seoul in the middle. So the plan is for her to buy LHR->HKG (then leave big suitcase in HKG), then HKG->ICN soon/the following day, we then have a 4-5 days trip in Seoul, then she completes the rest of her journey of ICN->HKG, stay for another 8-9 days then HKG->LHR. My wife and I would just buy a set of date-matching return ticket for HKG<->ICN. I hope this is clear. Not sure if I overthink the whole thing but I would like (1) no missed flight for her to ICN, and (2) the ability to leave a big suitcase in HKG while we go to Seoul.