Either a R3 or the NHT option would remove the small planes from the 2 main LHR runways
Neither of those scenarios is consistent with your 125 mppa, 480K ATM LHR.
If R3 went ahead, it would inevitably be accompanied by a hike in the ATM limit, otherwise there would be no point in building it.
If no R3/no ATM increase, then the only way of getting the average aircraft size to anywhere near the figure that you imply would be to shift all narrow-body ATMs (around 300,000 pa) to Northolt, which is clearly impossible.