The main difference between them all is that Optimax/Vpower; Caltex 98 and Mobil 8000 are all made by putting essentially octane booster additives to ULP or PULP95; plus then the "detergents" they all have (BP included).
BP Ultimate is refined to 98 octane then detergents added (ie no post refining octane boosters added). Note also 98 is a certified minimum, but fuel does deterioate with time, so you ideally want to fill up when a truck has just delivered to your local servo as often it'll actually be closer to 99 octane to allow for deterioation during tank storage at the servo.
BP would thus to most eyes be considered better as it's refined to 98. The others presumably dont do this because it costs more!
In most markets, there is some sharing, but it is not universal. Eg. in Brisbane, where Shell and Caltex both have refineries, they will sell each other fuel when maintenence etc needs to be done, and sell to non-refiners in that marketplace (ie. shell or mobil - but it's made / additives to the purchaser's specification) but they do try and do their own. Any extra they can sell to each other is bonus profit.
So it's not quite as simple as "it's all from one" in any one market - there's a whole mixture of things happening!