Don't stick a different back-box on just for a "chavvy" sound. I'd do the whole system from turbo-back - slightly larger bore and sports-cat and you'll get much better results for your money. (Power and sound)
 
fitted a scorpion cat back, it has noticable bottom end torque differences, thats it, no over all power increase to be honest, 2.5 to 2.75 cat back,