Save the money and go to Warwick. At Warwick try to study as hard as you can so you will graduate with flying colors and then pursue grad and postgrad studies at a top US university for economics such as MIT, Princeton, Stanford, Berkeley, Chicago, Yale, and of course, Harvard. There are Warwick graduates doing grad and postgrad studies at those universities and many of them are doing extremely well academically. Alternatively, you can pursue MBA at any of the top 10 business schools world-wide.
Duke is a fine school. But I think it is overrated. Unless you want to work in the US, go for Duke. Otherwise, go for Warwick and save the money. Warwick is just as good as Duke in my opinion.