It wasn't just number of aircraft. While not entirely due to the armored deck, RN carriers also had far smaller fuel resources available, both for aircraft as well as bunkerage. This was part of the basic design compromise that every warship designer has to live under and ships are built to reflect their mission and the strategic perspective of their country.
RN designs (not just the carriers, but other capital ships as well) were actually built with the idea that they WOULD operate inshore, hence the armored decks, but also with the clear understanding that the Royal Navy had a vast archipelago of bases circling the globe where fresh supplies would always be available. That didn't survive first contact with the enemy, but it was still a reasonable assumption to make since it had been the truth for around 300 years. The British ships were also built to fight in the North Atlantic, with the Med being more of an afterthought, something that made them very difficult ships in the tropics (all ships were something of roasters, but the RN had even greater difficulty than the U.S. or, remarkably, IJN ships). The heat vastly reduces personnel endurance and effectiveness, and by published accounts from the era, BPF personnel suffered more than their fair share, even with a dedicated brewery ships as part of their fleet train (!).