Heart Rate Wristbands
Heart rate monitoring sounds like the best feature ever, but there are different kinds of heart rate monitors, and frankly, some people don't need it at all. A built-in heart rate monitor drives up the price.
Optical heart rate monitors are the ones built into the device itself. The Apple Watch Series 2 has an optical heart rate monitor, as does the Fitbit Charge 2, among others. The Charge 2 can read your heart rate all day long. Other trackers, like the Apple Watch, only take your heart rate while you're working out. They are meant for heart rate training, not for watching your resting heart rate.
Some very good fitness trackers don't have a heart rate monitor built-in, but can pair with a chest strap. Most every device from Garmin and Polar supports a chest strap (like the super Polar H7), and you can usually bundle one in when purchasing a tracker for an extra £40 or £50
Finally, if you're interested in knowing your resting heart rate, you don't need to buy a tracker with an optical heart rate monitor to find it. Many smartphone apps, including Withings Healthmate, let you take your heart rate in about 15 seconds using the phone's camera. Check your pulse once or twice a day, and you're good to go