1. If we have US aircraft carriers - they'll have to constantly go back to the US to be worked on. Every time a ship comes back from Sea, they do some work on it - and there isn't much in the way of UK military ports that'll take a ship the size of a US aircraft carrier.
2. Cats and traps involve throwing a very heavy aircraft at a very high rate of acceleration (or deceleration) off a carrier. It isn't just a bit of welding, and we'll stick these on here. Since we don't have a nuclear engine to generate steam, it'll have to be EMALS to launch. EMALS is so brand spanking new, that the Yanks have just started to get it to work, and not even on a boat yet.
3. F-18 is an inferior aircraft to both Eurofighter, & F35. Indeed, on of the strongest areas of the F-35 is its sensor package, which is really useful fighting the kind of wars we are fighting, and will be fighting.