Easiest way, if you don't mind not having the latest content, is to use the Account Management site and use this code - FREE-UOKR-REG1-14DT

That code, I believe, gives you Samurai empire access
If you want the latest stuff though you'll need to either buy an upgrade code from uogamecodes.com, or buy the game online (Not sure if you can find it in the shops)