If you go through Quidco and search for Thornton's, you will earn 8% cash-back.
For only £5.95 you get 740g of quality chocolates, but it doesn't end there, if you put in a discount code you will get an extra 15% off *Terms & Conditions Consumers receive 15% OFF orders over £30. To be eligible for the offer consumers must quote reference code HWN7 in the promotional code box at the website check out page.
You can get 6 large boxes for Christmas presents for £33.84 which included P&P. (around £31 with quidco)