I think the British (and the Germans, the Russians, the Italians and the Japanese) were well aware of the need for a long range escort to do sustained bombing. The Brits had been bombing Germany since 1940 and they knew how fraught that was for bombers. The Americans had their very heavily armed aircraft and their Norden bomb sight and they had that faction we call the "Bomber Mafia" and they thought once again they could take a quantum leap past everyone else and defy the hard won lessons of their Allies. But they were proven wrong, and I doubt many in the RAF were exactly amazed when they were.
The problem was a matter of priorities - as I said before, interceptors and defending the homeland are really the #1 mission of the RAF, and the one they did best at- and the puzzle that is aircraft design. No one fighter could be all things. Planes great for escort did not necessarily excel at interception and vice versa. Planes great for high altitude did not always do as well down low. And so on. Even more interesting, planes that did well in cold weather maybe didn't do as well in the Tropics.
It took a long time to design a new plane and get it into action. That is why sometimes more incremental improvements like the Hellcat or the Mustang did better than more ambitious or radical designs like the P-38 or the He 177. The biggest achievement of the Hellcat for example was the rapidity with which they got it into action (first flight in June 1942, first combat deployment in March 1943). You could say the same about the Anglo-American (Merlin 60 series engined) version of the Mustang which I think is clearly a different plane from the American original - it proceeded from successful experiment in Oct 1942 to mass production (June 43) to flying missions (Dec 43) in quite rapid order. Something like say the Me 262 looks like a war-winner on paper but going from first flight in 1942 to arriving in the field in 1944 makes it less impressive to me than the Fw 190, which went from first flight to deployment in little over a year and got into the war in time to make a difference (if not to actually bring about victory).
The Japanese, as we know, started the war with the excellent, lethal Ki 43 and A6M, but fell behind in speed and performance, and their engine industry wasn't up to the task of creating a competitive fighter in the mid war years, so they didn't get their next generation killer warplanes into action until way too late. Most spent their service life in mostly futile efforts to stop the slaughter of the B-29 fire bombing raids.
You never know if a new design is going to work out, or which of many designs you start experimenting with will come to fruition... and which will suffer delays that prevent it arriving in time to matter. It was a crap shoot in many ways, and the way it ended up the British had an excellent interceptor from the start, thank God because it may have spelled the difference from the Nazi's winning the war. The Americans and Soviets had some 'pretty good' planes in the early war but no world beaters, and had to scramble to get some good fighters into the field in late 1942 and 1943. The Germans started with the excellent Bf 109 which started out dominant and stayed competitive, though always limited by short range, through the end of the war. To this they added the Fw 190 early enough to matter, but luckily for us all their incremental improvements to both were not enough to stay ahead through the tipping point of the war and none of their more radical designs really panned out. The jets got delayed and never came in enough numbers, the Me 309 didn't pan out, & the Fw 190D arrived too late.