You might find a local independent trader who'll take them but you'll never get the big supermarkets to do so. Here's why:
Firstly, no trader (big or small) will simply give money away, A trader will only accept a voucher if he knows that he can reclaim the money from the manufacturer. If a trader claims the money from a manufacturer, when the customer has not purchased the product, he is committing a criminal offence under the Theft Act.
The guy who runs a corner shop might be prepared to break the law in order to encourage customers into his shop. As long as he only accepts vouchers from a few regular customers, it's unlikely that the manufacturer will be suspicious when he submits his claim for a refund of the money. Even if he gets found out, it's likely that the manufacturer would not bother to prosecute. They'd simply refuse to pay out and inform the retailer that they won't refund any vouchers he sends to them in the future. The shopkeeper's trade won't be affected; he can continue buying his stock from the local cash and carry.
However, if a major supermarket chain adopts a policy of accepting the vouchers (without an appropriate purchase) and then seeking a refund, the manufacturer will seek both criminal and civil action against the supermarket. The resultant fines, damages, court costs, legal fees and bad publicity could cost the supermarket chain millions of pounds. Additionally, the manufacturer could refuse to supply the supermarket with any of the brands it manufactures. The supermarket would lose the profit on these lost sales for years to come. It's also possible that shoppers would get fed up of shopping in a store which didn't have their favourite brands and start doing all of their shopping elsewhere. The total cost to the supermarket chain could run into tens of millions pounds.
Chris