Lumia 900 global version is now on pre-order at Amazon and Clove in UK and on KPN in Netherlands. Release dates vary from 14th to 16th May. The best price seems to be on KPN with Euro 39.95 on 2-years contract. Amazon has put pre-order price of £479.99, while Clove beats it with pre-order price of £458.50.
The pre-order links are given below.