You can get it on Orange pay as you go, however you cannot switch as it is locked to the network.
But you can always buy an unlocked model, which will accept any sim card.
Orange: http://www.play.com/Mobiles/Mobile/4-/3595686/Sony-Ericsson-W580i-Orange-Pre-Pay-Mobile-Phone/Product.html
Unlocked: http://www.play.com/Mobiles/Mobile/4-/3595129/Sony-Ericsson-W580i-SIM-Free-Unlocked-Mobile-Phone/Product.html
My freind has the phone and it is pretty good. It is basically the walkman version of the S500i, with the Shake Control! Good luck.
P.S You can also get the orange phone unlocked. Usually around £10 to get it done.