Hi danny.
Roughly, the air we breathe in is 80% nitrogen, 20% oxygen.
The air we breathe out is 80% nitrogen, 16% oxygen, 4% carbon dioxide.
There is currently a great deal of controversy about the effectiveness of expired air resuscitation and it's my bet that very soon The Resuscitation Council of GB will say that it's not needed. Current first aid training still teaches it, but if, for example someone has severe facial injuries and it cannot be administered, chest compressions can still be effective at circulating oxygen around the body. (The lungs always retain a certain amount of air, called residual volume).
The only time I would say that "mouth to mouth" is essential is in cases of drowning and carbon monoxide poisoning.