Thorntons currently have an offer on some boxes. I think it's 4 for £20.
My MIL actually got a huge hamper of thorntons off QVC last year to split for gifts. It worked out very cheap. Was full of large boxes of all varieties of chocolates, medium boxes of their continental chocolates, boxes of toffees etc.
she managed to give a box of chocolate or toffee to everyone in the family as a stocking filler. For a lot less than she would've paid for actual odds and ends to be stocking fillers.
Not sure if they will do it this year, worth looking?
Otherwise, close to Christmas asda often do the 3 for £10 deal on their milk tray/dairy box eye double layers. X