The How-To-Not-Get-Your-Bike-Stolen edition
Just buy one of these:
The Kryptonite Series 4 is a solid all-rounder. It’s around £30 on Amazon so it isn’t going to break the bank, but that gets you a reassuringly solid (and heavy) feeling lock. There’s a bracket so you can mount the lock to your frame to let the bike take the weight instead of carrying it in a bag, and the mount works well with not too much rattling.
It locks at two points (which is apparently good), and has a Gold (the highest) rating from Sold Secure (a British rating scheme run by locksmiths), which sounds good too. Often you’ll need to have been using a Gold or Silver rated lock to be able to make a claim against a bike insurance policy.
You get three keys, including one fancy one with a built-in LED light.
The lock is bulky, but that won’t matter when it’s mounted to your bike’s frame. It’s long, which is useful for locking bikes to things in awkward positions, or by locking both your bike and a friend’s bike to the same railing.
If the lock looks too big for you then there is a smaller version (the Evolution Mini 7) (£27), and then a really-smaller version (the Evolution Mini 5). I use the Mini 5 for one of my bikes and it’s nice and compact, but it really limits what you are able to lock your bike to. The Mini 5 often can’t extend far away enough from the bike frame to fit around something like a lamppost. Here’s a picture of the Mini 5 to give a bit of context:
Whatever you do, don’t buy a cable lock, or you’ll need to come back to this post after your bike inevitably gets stolen and you have to buy a new bike and a new lock.
As well as what lock to buy, make sure to read up on the best way to use your bike lock. This page is pretty useful, as is this one from Lifehacker. If you want to know more about bike locks that you’d ever need to know, then check out thewirecutter.com’s article on the best bike locks to buy.