2 observations
1) If you see something identical to what you bought e.g. last week and now there is 20% off, it's easy. Buy another at the reduced price, take one of them back with the original receipt and get the "full" refund. I don't see any ehtical problem with that. (Provided you return an unworn new item.) OK Gamma in your case not available. which was unlucky. One trouble is that popuklar items sell out very quickly as stores do not carry as much stock as they used to.
2)I bought the jumper shown in their Christmas commercial on the 20% discount (online) , decided I wanted the next size up by which time it was full price, but got a straight swap without paying the extra as I explained I had bought online and it was the wrong size.