That's a moot point, because at LHR we do not give the QNH out at all, either with pushback or taxi.....if you state the correct QNH first time 'round.
The reason? So we can ensure you have the correct QNH. I'm having problems understanding what is so difficult to grasp.
Gonzo, I don't frequent LHR these days (but did so in a previous life when people boarded a/c wearing suits!) but if you come north of Watford you'll find that at certain unmentioned airports, depending who's on duty, we get the QNH rammed down our throats several times even when it hasn't changed or we give it on initial call with ATIS X only to have it given to us yet again! Am all for safety and anything that avoids CFIT and/or Altitude Bust is a good thing but if you cry wolf too often it loses it's effectiveness!