I haven't tried it myself but I did read online somewhere (from a forum on British 'Extreme Couponing') that British companies don't tend to be as free with sending out vouchers as American ones are, especially if you're messaging them solely to ask for vouchers and that you're more likely to be sent a free sample or similar.
Worth a go, though!
If I ever have dissatisfaction with a brand then I'll email and then they do tend to send decent enough vouchers as compensation, but I guess usually that means you have to send them information (barcodes, dates, store location etc.) about the faulty product.
I'd be interested to hear how you get on with it though!