Where does the English accent come from?
The simple answer is that it comes from Coruscant, and it's the Star Wars galaxy's equivalent of Received Pronunciation (the stereotypical English accent). It's the accent of the educated and upper crust in the Empire.
The question makes some rather odd assumptions, though - sure, there's no planet "England", but there's no planet "America" either, yet we hear American accents all over the place. Indeed, if anything it's the American accents that need to be explained, since we hear them from individuals from vastly different backgrounds!
In fact all the English in the films is a 'translation' of Galactic Basic, and different regional accents are used to differentiate groups from one another. It's a simple filmmaking technique.
Why did a young clone have an English accent?
Without specifying which young clone you heard with an English accent it's hard to say for sure, but I suspect the issues is that you misidentified a New Zealand accent.
The voices of the young clone troopers were provided by a New Zealand actor named Daniel Logan. He was pretty much the ideal actor for the role, since he also played young Boba Fett in Episode II. The actor who originally portrayed both Jango Fett and the clones, Temuera Morrison, is also from New Zealand, and both actors used their native kiwi accent for their roles.
In the Clone Wars series, the voices of the clones were provided by Dee Bradley Baker doing his best to imitate Morrison's accent.