As I recall with the Queen Elizabeth class, the Ministry of Defence decided to go with 36 F-35Bs for peaceful operations instead of 48, but they can at a later time choose to go with 48 F-35Bs in case of a war. Having 24 or so extras on hand which can be flown by reservists can be handy during a crunch. Face it, by the time the Lightning IIs enter service the Harriers will be pretty much worn out unsuitable for reserves. The same can be said of the air force's squadrons. With the joint force, having a reserve of extra aircraft won't hurt for either service. While this appears a bit too much, as noted, it is a wise policy. While the extra aircraft won't squeeze in the hangar, they is plenty of flight deck space aboard the two carriers. I see no problem with the joint force, the US Marines fill out US aircraft carrier air groups all the time. With future upgrades in mind, having extra aircraft will make it easier to do the upgrades without suffering aircraft shortages.