Amazon Prime Wardrobe - Try Clothes for FREE
If you are a Prime customer, you can choose between 3-8 eligible items (found in the link in the OP) and Amazon will send them to you without charge for 7 days. You can try all items on, and use the bag and prepaid Royal Mail returns label to send the clothes back free of charge.
Furthermore, if you keep between £100 - £199.99 worth of items you get £5 off. If you keep £200 or more worth of items you get £20 off! More information here