BOAC, a word of caution on Vref +15 at touch down; I understand that the performance certification assumes Vref +15 as the max speed at the threshold; perhaps this is what you meant.
Also, to remind us all that landings at Vref +15 (from the threshold) do not guarantee stopping in a declared (limiting) distance, particularly where the performance margins are degraded (brake / tyre wear / operational technique) or runway conditions are less than ideal (wet – verging on contaminated / at near max wt). Most of the assumed margins can be quickly eroded by crew judgement (or lack of judgement) e.g long / fast landing, failure to apply full brake, misjudging a ‘wet with water patches’ runway as a wet runway, and assuming dry performance always works on a wet grooved runway. As ever, it is safer to arrive at the correct place at the correct speed.
Re bug setting. I hope that the time at which any speed additive is reduced in gusty conditions is at an altitude where reference to the approach speed bug is no longer required (predominantly visual flight). Alternatively assuming that you have the luxury of dual bugs, the final speed checks can be made against that value to be achieved at the threshold (set on the second bug); the monitoring pilot also uses this as the final reference.
This could open a debate about minimum approach speeds in turbulence. One of the reasons for using speed additives is to mitigate the effect of airspeed fluctuations. If speed temporarily deviates low, then the limiting condition is the min Vref for the aircraft configuration / wt, as is still the min safe speed for landing; the limit is not the higher bugged ‘target’ threshold speed; this is based on the gust being transient. However if a long term speed error is introduced then the bugged (increased) threshold speed applies in order to retain some gust protection. I see too many pilots using speed additives as absolute values and not as speed targets.