Is it theft to buy pairs of shoes in different sizes and return one of each? England

I have two different sized feet, roughly 10.5 and 12. if I wear a size 11, my right foot suffers from cramps and rubbing on the toes. if I wear a size 12, my left foot is like a hotdog in a hallway.

buying two pairs of shoes and throwing out one of each pair is expensive. So if I was to hypothetically buy a pair of 10.5s and 12s from the same shop for the same shoe style, then do a return where I give back the 10.5 for the right foot and 12 for the left in the same box, have I committed theft?