Remember "Afghanistan" is the "last war" - a defence review has to prepare for future wars.
If we lose,or are seen to lose in Afghanistan, there will be no future wars. Just small scale interventions with no requirement for a carrier.
Look at the USA post-Vietnam. Grenada, Panama, and finally Somalia. It took 9/11 to break them out of the defensive stance.
I'm not against 2 new carriers, but lets put resources where we know they are needed NOW, not navel (pardon the pun) gaze and second guess the next 25 years.