Visa is waived for UK passport if you have an outbound ticket. If this is one-way, you will need to show some other outbound ticket showing that you are leaving Hong Kong.
The Most Significant Carrier rule applies to the contractual ticket, not to elite privileges. As OS is not part of OW, I doubt that they will give you extra allowance based on your AA status.
I have no idea what OS standard allowance plus *A Gold would give you.