BA back up their operations very well with standby, available, ring-in and QRS shifts.
In addition theres always crew queuing for overtime.
BA have had a 747 available on standby for the tsunami releif runs.
BA are also in ongong recruitment for eurofleet and as a result are transferring existing eurofleet crew over to worlwide at a pretty fast rate to cover retirements and future operations.
Some airlines have a policy of only following the minimum JAR-OPS crewing levels. BA follow the JAR-OPS minimum also, but also crew their aircraft depending on passenger loads, so will call out more crew for an operation if there is a heavier load. I've never been on a flight yet which has had problems with crewing.