Bike theft can be big business. The police have limited resources and 1 bike stolen is not a major crime, but if you could steal 5 a day and sell them on for £20 each that's easy money. No initial outlay means 100% profit. That's if sold locally. Posted online, Gumtree or ebay, a decent bike could sell for hundreds.
So, how do you deter your pride & joy from being chored by a sticky fingered, light footed ne'er do well. The most secure option is to keep it indoors if at home and take it into any building you visit when out. Unless you own a folding bike this isn't practical. Which means leaving it outside.
Bikes are often left unlocked outside chip shops, bookies, newsagents, coffee shops and post offices. Granted many of these are of low value and in rural areas ( where theft is rare), but they still cost money to replace. Many of these thefts are opportunist and even using a lock from a pound shop can mean the difference between riding home in 10 minutes and walking home in 1 hour.
Don't lock the bike by just the front wheel, quick release ( q/r) levers mean you come back to find a very secure front wheel only. Using two different types of lock makes it harder to steal ( different tools are needed to cut through them ). A cable lock running through the wheels ( especially if q/r ), attached to a D-lock which is locked to a large immovable object . Don't lock around a 4' high bollard. Leave as little room as possible between the bike and what it's secured to, this prevents the use of poles used for leverage to pop the lock. Remove the seatpost if q/r or replace the q/r with a lockable bolt. Remember to remove any valuable equipment such as a GPS, high end lights or ebike batteries. Leave it locked in a high foot traffic area. Preferably near a cctv camera and under a lamp post. Never leave it unlocked in a secure stairwell, the service button for the postman and deliveries means everybody has access.
If locking in a bike rack, leave it next to a bike with a cheap, easy to remove lock. Why risk spending minutes to cut a lock if you can get through one in seconds.
Bike insurance is an option, but without the use a lock approved by the insurance company ( covered here soon) and A) proof that it was your bike B) secured in a well lit area and C) with a police report of the theft, it's unlikely that any recompense will be forthcoming.
Take photos of your bike, taking note of any parts that you've replaced ( making it unique ),large dents or deep scoring ( impossible to hide) and the frame number ( usually stamped into the underside of the bottom bracket shell).This info will be helpful if it disappears. Inform your local second hand bike shop.
Finally, if buying a used bike, ask for proof of ownership or receipts from it being serviced ( anything that proves it is legitimate). Be wary of bikes with parts fitted from multiple brands e.g. Trek branded bars with Specialized stem, Giant seatpost , Carrera seat and mismatched wheels. "Heinz" bikes are often built from multiple stolen bikes to make identification difficult. If an £800 bike is being offered for £150 and it's in mint condition then walk away.