Non-existing exchange policy

I bought sneakers online which were on an extra 25% on 29 May 2024 and chose the option of collecting them at Southgate Office London. Because I was not fami**** with the brand, I bought my normal size 7. The order was delivered at Office London on Monday and I got the message and attended to collect my order on Tuesday. When I fit the sneakers, I realised that I needed to take a smaller size. Fortunately Office London Southgate had a size 6 but when I requested an exchange I was told that I would have to pay the current price as the sneaker was no longer on the extra 25% sale. I had to return the sneakers because keeping them would have been pointless while I could have just exchanged them for the same brand just a size smaller. I have been searching for this exchange policy but cannot find it anywhere because an exchange is just a replacement of a similar product. The shop was very much aware that I am only getting the shoes right then so I couldn't have exchanged them sooner. I am really frustrated and disappointed by Office London