Running watches can range from £50 to £600+ and while usually, the more you spend the more features you get, not all features will be useful to you and your running.
Running watches within £100 will come with limited features, sometimes without GPS, but can pair with your smartphone for added data. These are usually good lightweight options giving you pace, distance, and calories.
Watches at this level should come with heart-rate monitors, GPS, and built-in accelerometers so you can record accurate data whether you're inside on the treadmill or outside. You'll be able to utilise heart-rate zone training and interval timers for ultimate performance training.
At the higher price point you start to get the really interesting features from vertical oscillation, to a built-in compass, multi-sport tracking, and real time coaching. Watches at this level will offer VO2 max and even be able to calculate your stress levels.