Winter schedules are not finalized. I don't think WS knows what it's doing with the 787s come winter but I rather doubt it returns to 763 service. Likely 2-3x a week to LGW on 789s. Patience.

The 789 is an upgauge (320 vs 260) and has a business cabin. So they are not at all close capacity wise.

Airlines change equipment all the time depending on the market and where they can best profitably use their assets. This is not false advertising as you can see your aircraft and service class before you book.