I think they probably would refund to gift card.
I did similar at Next before when I picked up some cheap gift cards (although I had genuine reason for returning the item rather than trying to make money on the gift cards )
... But yeah they refunded it to new gift cards.


Oh and yes you can use on tesco.com, I used some of mine tonight on tesco direct.. It's a bit of a pain scratching off all the PIN numbers but I suppose I will have to live with that