Changes in the law now mean that you must register your drone (or model aircraft) if it weighs more than 250 grams. You will need to obtain both a "flyer ID" and an "operator ID". More information on both of these mandatory requirements can be found on the CAA website.
Click here to access the CAA registration service.
The CAA website will also guide you on what else you need to do if you are planning to fly a drone/model aircraft for commercial filming purposes.