If BA cancels the flight, they will be very helpful about getting you onto another plane, even on another carrier if necessary.

But they won't do this UNTIL they have actually cancelled your flight.

A contract exists between you and BA from the moment you pay for your ticket, under the terms of which BA has an obligation to use its best endeavours to transport you from A to B. Interestingly, it's not an obligation to transport you by air, so in theory you could find yourself riding from Sydney to London by rickshaw, and BA would have fulfilled its obligation to you!