The Earth is still round: You still see further from high up
That issue is the provision of a carrier airborne-early-warning (AEW) radar aircraft. If a fleet has no such aircraft overhead, attacking planes can approach at low altitude, hidden below the horizon with respect to radars mounted in the ships. By the time they are detected, it will usually be too late for patrolling fighters to intercept them before they strike - and often too late to do anything effective at all.
A few of these and the Falklands would have gone a lot better.
In the Falklands the Royal Navy had no AEW aircraft, and Sea Harrier fighters had to be in the right place by luck in order to repel Argentine airstrikes. They did sterling work, but far too many attacks got through to the ships; and losses were heavy as a result.
As a result, the navy now has AEW helicopters, which are better than nothing. But a helicopter can't fly as high as an aeroplane - hence the radar can't see as far - and it can't stay up as long either, or roam as far from its carrier. (This last is important, as the search radar acts as a massive radio beacon to the enemy - it's nice if the AEW platform can be well away from the ships.)
Under the current plans, the RN will probably have to carry on with radar copters, or perhaps in future a specially-fitted V-22 tiltrotor (an option unfortunately but perhaps accurately dubbed TOSS). This will be expensive, as it involves a small fleet of custom aircraft, and not really very capable.
But CATOBAR ships would allow the purchase of E-2 Hawkeye AEW carrier planes, as used by the USA, France, and also many other non-carrier nations (as an affordable alternative to massive land-based AWACS radarcraft, in their cases). There's a new version out (the E-2D) for the US Navy, or there's the much cheaper option of buying variously spec'd earlier E-2s as several countries have done in the last few years - perhaps a good idea for today's cash-strapped MoD.
Air groups of F-18s and E-2Cs would be billions cheaper than ones of F-35B hoverjets and TOSS tiltrotors, and in many respects actually better. They could be enhanced easily with pricier F-35Cs and E-3Ds down the road in future.
So what's the catch?