Basically - imagine you are at the airport you are supposed to depart the UK from; let's say at the scheduled departure time you are told "cancelled"....can you work out how to still get onto another flight to 'position' in time (perhaps on BA, or another airline...even another London airport) - could you make it?

You would then have to decide how far into a "delay" you can run before you implement the back-up plan.

If there is no back-up plan possible, then you really are in the hands of the travel gods if your flight is cancelled...