Anyone who followed YTV during the early 2000s might recognize Britney's second voice, it's YTV host Stephanie Broschart!
Britney's first voice actress was Lindsay Ridgeway from Cats Dont Dance, but it's been six or so years since the days of Darla Dimple, and thus her normal voice is unrecognizable as the little girl who belted out "Big and Loud", reducing the effect of this.
The fifth season has aired just about everywhere but the USA.
They did air it here. Unfortunately it was over that one summer, and they haven't shown it again since
So far, The Movie doesn't seem to have aired in Canada. And we don't get the first season anymore, only the Undercover seasons, as far as this troper knows.
Rectified a bit when the fifth season and movie finally aired on Cartoon Network in the USA in the spring of 2010. Still seems to be scarcely appearing in other territories, though.
The Other Darrin: To put it simply, the English dub switched from American voice actors in the first two seasons to Canadian for the rest of the dub. Clover and Sam's original actresses were retained for the entire series, though, and Word Of God has said that they wanted to retain Alex's voice as well.