You can't if it was a custom order. A quick google of their policy tells you what you need to know.
Which items can't be returned?
We want our customers to be happy with their purchases and aim to be fair and accommodating with regards to returns. But unfortunately, some of our items are non refundable (unless faulty). These items are listed below:
Made to order items, food & alcohol gifts, and flowers.
For hygiene reasons, all underwear and swimwear products that no longer contain the hygiene seal (such as individual briefs, bikini briefs, female swimsuits and men’s swimming trunks and briefs), opened packs of men or women’s underwear, all cosmetics and grooming products, certain jewellery, (for example pierced earrings), mattresses, mattress toppers, divans, duvets and pillows.
Self-assembly furniture cannot be returned if assembly is part or fully completed.
Any product with a security seal, (such as CDs, DVDs and other technology products), cannot be refunded or exchanged if the seal is broken.
Gift Cards and Vouchers cannot be returned, or exchanged for cash.