DVLA is all setup for in. easy online to find the guide.
if converting body types, you may find this causes you to have to pay the low emission zone if you live / drive / work inside the london zone. There is an exception for station wagons with seats fitted that double cabs don't get. Also you'll have to pay £14 to cross the seven bridge rather than £6.
I've got a station-now-double-cab. And have had to discover this.