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Here is an interesting site that explores this issue:

"Magwitch may have earned his fortune by honest labour, but because he has done it in 'the vulgar colonial fashion' (Sadrin in Great Expectations), he can never be accepted into English society where standards are more demanding. The lax morality and classless system of the colony can not be transferred to the centre, at least not yet - therefore Magwitch, though he may achieve success on the periphery, cannot hope to be welcome in England - and Pip, by his association with 'colony money' will never be a true gentleman."