The calculation is average CPI over the 12 month period capped at 4%. That’s how the 2.4% was calculated. Some rules may be higher. I don’t know of any administrators who wait for that order tbh and have worked in a few companies that do pension scheme admin.
I manage a pension scheme and our admin team apply the revaluation as soon as we have the calculation in third week in January. The revaluation has to be applied, so even if pension goes into payment before revaluation is applied the next monthly payment would be adjusted and back dated to when pension commenced.