A couple of route change suggestions that take you on less busy roads
At around 176km, just as you descend, turn right into Pound Lane follow until mini roundabout and turn right over Marlow Bridge. (Costa Coffee on right and public toilets). You miss the busy Marlow High Street.
At around 194km turn right@ TFL towards Beaconsfield Services and then take 1st exit off roundabout onto Hedgerley Lane. Follow for 2.5km and then turn left @ T, follow for another 2.5km and turn L @ T and follow road back into Gerrards Cross. This avoids crossing the M40 roundabout and using the A40.
Neither reduce the length of the ride or the amount of climbing.