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Cover, biggest chain you can afford and a disk lock. A few burly blokes with scaffold poles can still whip it into a van so chaining to a ground anchor or lamp post etc is best.
I use an Oxford storm cover - http://www.oxford-shop.co.uk/item3714.htm
Pragmasis lock and chain - http://securityforbikes.com/proddetail.php?prod=P19-x.x-SS65CS
Abus disc lock - https://www.mcaleicester.co.uk/accessories/security/disc-locks/abus-a6-disc-lock/
Security for parking on the street. What's essential?