Arctic cable is good, but it is still PVC. Even better cable for extension leads is rubber, to HO7RN. These cables are really easy to coil up - they don't end up looking like a snakes wedding and easy to uncoil. If you are using a long extension, then it should really have 2.5 sq mm conductors, even though 1.5 sq mm is rated at 16 A. I have used this site for cables:
www.toughleads.co.uk.
I have several cables between 10 m and 25 m long, with 16 A commando connectors on both ends, then I've got UK and Schuko (EU) adaptors for the supply end and a 4-way UK socket adaptor for the other end. If I need a long lead I just connect them together and make sure that the connections won't end up in a puddle if it rains.