If this is a firm decision, just let the school knows.
My advice is to notify the school by yourself in person well ahead (that is before the school gets known from the UK school) that (1) the school would feel being respected; (2) the school can plan ahead better for a good report.
In my case 2 years ago, I informed the vice principal of my son's school when I was notified of the reference report. I told her it was our plan and if he did not go for Year 7 (if not accepted by preferred schools), he would go for Year 9.
For the school to produce a "helpful" report, you may have to provide additional information, in particular, regarding those extra curricular activities. The more information you provide to the school, the better and meaningful report it would be.
For the FT top 100 schools, they may look for students with good language ability in English. For some schools, they don't have a separate Eng entrance paper for non-native students. Hence, the result is usually not as impressive as natives (my son told me there was content of Eng literature). To add comfort to the target school, a report from the existing school illustrating the language potential and ability of your child is to some extent important.
Besides, British are relatively open on this. My son's school sent a notice to parents in mid of year 7 and asked if there was intention to further study in other senior schools for year 9. The headmaster even offered for discussion, if need. So, it would be quite odd if the HK school replies they are not aware of.
My understanding is not many schools request for reports from existing school, mainly those high ranking ones. These may be obviously your targets. Why not make a better start.