When one of the big HK conglomerates decides not to buy franchise rights or set up a JV for all of HK as is done for McDonalds and Starbucks, then that should tell you that they did their due diligence and passed on Subway as something viable (long run economically feasible). Exception would be Subway prefers individual operators but this is not the norm for franchisors.
Do you know the payback period, historical store comps, etc. for a typical Subway in HK?
Also, Pret having to scale back globally due to COVID but also seems a tired brand. In HK, my perception is outside of Central, concept doesn't work. Also, changing demographics of HK workforce (expats from Mainland supplanting Westerners) is writing on the wall for selling sandwiches IMO. Locals will eat a pizza but sandwich perceived as rubbish peasant low class food. Same for Mexican food IMO.