After searching around for Wool Mattress Topper found this one, which is amazingly cheap(they usually cost around £100-150 for King size). The price is £50 plus shipping £5.95.
The only size they do is King.
Why wool? One of the answers: It warms you almost immediately when you lie on it, and you won't find any random cold spots, and frankly, the total feeling is so snugly wonderful that you just won't want to ever leave it! On wool, nature's miracle fibre which is naturally hypo-allergenic, dust mite free and virtually flame resistant, you will rest comfortably and safely. Wool has its own climate control, keeping one warm in the winter and cool in the summer. Unlike most synthetic materials, wool breathes enabling it to absorb moisture without feeling wet to the touch.