When a retailer, for a single transaction, offers a simple receipt for accounting/credit-card purposes and an optional VAT receipt, they are obliged to ensure that the simple receipt cannot properly be used to reclaim VAT. If a customer tried a double-reclaim using a mutilated simple receipt, I suspect HMCE would come down on them rather than the retailer.
If such a receipt bore the VAT rate then the situation would be more interesting, and HMCE would have grounds to penalize the retailer; but I suspect that you won't find any examples of that.
Matti