Yes, an England-only team of Appleton, Peach, Melling, Gray and Boyes would be very strong, but, as another poster noted, having this competition as America vs Europe brings much more positive attention to the game.
I've heard it argued on the forum that it ought to be continent vs continent, Europe vs North America, but nearly all the best North American players are from the United States, so it makes little difference. I believe that Jason Klatt and John Morra, both skilled enough to be capable of earning a Mosconi Cup spot, are Canadian, so they are the exceptions.
Of course, there are way more pool players in America than in Europe, so maybe the point is moot after all.