The Nokia Lumia 800 White is available for pre-order at Phones4u in UK and it seems the for now the Nokia Lumia 800 White will be exclusive to Phone4u which has contracts available on O2, Orange, and Vodafone.
The Nokia Lumia 800 White is available for free from £20.50 a month on a choice of networks from early March.
More info can be found at phones4u blog post and the phones4u page.
