You'll need a lock if you leave your bike unattended when 'away from home', and the lock has to be approved by Sold Secure www.soldsecure.com
It's dependent on the value of the bike as to which lock rating you'll need:
Bike value under £250 - Bronze
Bike value under £1500 - Silver
Bike value over £1500 - Gold
The rating is based on the amount of time and the tools required to try to break the lock, so it’s always worth buying the highest rating if it’s in your budget. The higher the rating, the longer it took, if it was actually possible to break! There's some great deals out there, and you can often get a Gold rated lock for the same price as a Silver.
If your Sold Secure approved U lock comes with an additional wire cable, please be aware that the cable is not included in the approved rating, it's just the lock. This means that if you secured the bike through the frame to an immovable object, with the cable only, you wouldn't be covered!