I'm sure you could probably order it on the web for shop collection, get a refund then buy it again with the discount. That would be my approach, unless you've got a John Lewis master card. The partner discount is automatically applied, however as my wife is the JL Partner she has to get the card and add me as an additional card holder - but she doesn't want a Credit Card.
Option 2 for me is get my mate to buy it as his employer get the same discount (or close enough), my employer discount is around 7%, which is better than a kick in the knackers, but still not 12.5.