That sounds bad. No way should that external wall vapour barrier be damaged. Not only will you get cold air in as you describe, but there will be a real risk of interstitial condensation. An airtight box would stop the cold air reaching the room, but behind the wall surface there will be a steep temperature, moisture and dewpoint curve.
If the boxes or their fitting procedure is damaging the barrier then there is a design flaw; the architect has not provided for electrical outlets in the structural elements. With brick/block this isn't a problem of course and IMHO the real problem is one of concept. We still build a structure and then try to fit in the requirements for life, rather than the other way around. However, we must deal with the world as it exists! I see the problem of using a 50mm void, but what if the vapour barrier (or a reinforced patch) was capable of being made flexible with say a hot air gun and a box with a curved smooth back would help avoid fitting damage? Vertical cables would be needed, but this wouldn't be difficult or expensive although 90 deg drills would be needed.
If the boxes or their fitting procedure is damaging the barrier then there is a design flaw; the architect has not provided for electrical outlets in the structural elements. With brick/block this isn't a problem of course and IMHO the real problem is one of concept. We still build a structure and then try to fit in the requirements for life, rather than the other way around. However, we must deal with the world as it exists! I see the problem of using a 50mm void, but what if the vapour barrier (or a reinforced patch) was capable of being made flexible with say a hot air gun and a box with a curved smooth back would help avoid fitting damage? Vertical cables would be needed, but this wouldn't be difficult or expensive although 90 deg drills would be needed.