Basic question, but which type of cable should I be opting for for an extension lead.
Good question which despite the replies here so-far hasn't been properly answered.
There are two questions, size and type.
Size comes down to both the load and distance involved. The normal "default" size is 1.5mm² which is rated at 15A giving a little margin over the 13A of a normal plug and socket. Sometimes you also see 1.25mm² which is rated at 13A on commercial extension leads but this is rarely sold seperately.
For especially long or heavy loads 2.5mm² may be worth considering though it's a tight fit in a 13 amp plug.
Smaller sizes may be able to be used for lightly loaded leads but if you use these you need to reduce the plug fuse to match .
As for type you should not use cable intended for fixed wiring. Ordinary flex is fine for basic leads, "arctic" flex is tougher and remains flexible at lower tempreatures and tends to come in bright colors making it more easilly noticed so it's a common choice for tradesmans leads.
The theatre industry likes to use rubber flexes, these are very tough and flexible but don't last as long as PVC. They are usually black which is good if you want things to make the cables running through your lighting rig as discrete as possible but makes them more of a trip hazard if used loose.
Going beyond the cable you also want to select appropriate plugs and sockets. Ordinary brittle plastic is ok on short extension leads or ones that are rarely moved but if you are going to be throwing and dragging the lead arround repeately as you take it in and out then you want something tougher.