Buy knickers, vests, swimwear and beauty in a sale only in the exact size, fit and shade you already use from Marks & Spencer, since these categories cannot be returned.
Sale discounts at Marks & Spencer need no code, and the reduced price is applied in the cart or at checkout, so the price on a product tile may not be what you pay. The End of Season Sale puts different styles in different bands, such as flat 60%, 50%, 30% and 20% off, and the Mid-Season Sale uses bands of 40%, 30% and 20%. Add the pieces you want to the cart and compare their final prices there, not the banner headline.
Last Chance to Buy pieces carry stricter rules. Anything there priced at INR 990 or below cannot be returned, and some higher-priced pieces there are also non-returnable. Each product page shows whether that piece can be returned, so check it before you add it to the cart.
Some categories cannot be returned at any price: beauty, knickers and knicker packs, swimwear, shapewear, trunks, briefs, boxers, vests, thermals, tights and some socks (the product page says which). Treat any sale buy in these categories as final.