I do believe that you require a stamp in order to obtain a refund of your VAT, hopefully they will return the forms back to you and perhaps on a later trip, you can have them stamped. Of course, the drawback is that you need the goods on your person, so this may not help. When traveling in the UK and France, I’ve always had my forms stamped, allowing at least 2 additional hours for the line at CDG each time. For my upcoming trip in June, I’ll do the same and since I’ll be in London as well, I’ll ask at the counter if refunds can be obtained with out a stamp, though I highly doubt it.