It's only 99% of the artificially limited capacity. Under FAA rules the runways would be independent and run in mixed mode.
Mixed mode would add about 15% to runway capacity. It would arguably add nothing to resilience, since commercial pressures would ensure that movements would grow pro-rata and utilisation would be back up at 99%.
And the fact that LHR doesn't operate in mixed mode has nothing to do with the fact that FAA rules don't apply.