SIN, HKG & JNB flights are away from LHR for two nights so that's a maximum of nine daily departures. Allow for maintenance & cover and eight departures is the maximum for most of the year (possibly nine on some days in the summer). That allows for a maximum of three routes (Only one LAX rotation is operated by A380 in the winter.)
I understand that T7 at JFK cannot accommodate the 380 even though 1 & 4 can & do. I don't know about BOS.
I would have thought MIA in the winter would make sense. What about YYZ or even YVR in the summer? Could even be that HKG goes 2 x daily 380 at some stage (it has been 3 x 747 in the past).
Just my thoughts about the routes.