According to Three’s Best Sellers list, the’re a Nokia Lumia 800 on page 1.
Go through Three>Store>Best Sellers and surprise, it’s still there.
Also of interest, from Vodafone. Nokia Lumia 800 is at the best sellers page for business. Their personal section doesn’t seem to have a best seller option. Hoever, the Nokia Lumia is on the first page (top 5) for customer ratings and it has the most customer reviews in comparison to the other phones above it.