Originally, the book was written in French. The 2012 adaptation has English actors and they are expected to speak in English.
However, why do they speak with different accents?
I could get it if it were only actor accents (Daniel Huttlestone is British), and not character accents. But Thénardier's french accent is definitely a character one.
Are they using different English accents to demonstrate different French accents of the original novel?