I think that is precisely the distinction that many fail to understand. Britain is not free to field a baseball team. MLB would not allow it.
As for the World Baseball Classic? It's a joke, because the best MLB players don't play.
MLB would have nothing to say about it. Japan, South Korea and Taiwan (just to name three) all have very healthy professional baseball leagues in their respective countries. While they could not compete at the MLB level, the Japanese are at least getting very close.
I have personally met the Japanese Commissioner of Baseball in a previous life on a business matter and he told me they have a very good working relationship with their MLB counterparts. For instance, MLB will send an all-star or World Series champion team to play their Japanese counterparts every November or December. Every game is a sell-out and nearly every game is competitive, though the American side generally wins two-thirds of the games. At one time, the figure was close to 100%.