You need to go to http://www.ofcom.org.uk/licensing/olc/
, and click on the Register or Login link down the bottom.
You then fill in all your registration details, and Ofcom will send you your username and password through the post (!), enabling you to login, set up your portable license for free, and print it out.
Be aware that this license only covers the portable radio, not the operator, and you do still need to hold a Short Range Certificate or Restricted VHF Operaters Certificate to use it.
Hope that helps!