Given that you're in the UK (like I) you must be using EE 4G? If you're using EE (Orange or T-Mobile) then you're currently out of luck as neither of those brands currently support it.
To enable WiFi calling on your iPhone 5c:
Go to Settings > General > Software Update on your iPhone 5c. If you already have the latest version of iOS installed it will tell you. If not, choose the Download and Install option. Make sure your phone is connected to WiFi and a power source, and tap to download the update. You may have to enter your passcode before installing. You will also need to update the Carrier Version. Go to Settings > General > About which will trigger the update. Complete this step after the 8.3 Upgrade. You should see 19.1 as the Carrier file.
2. Connect your phone to WiFi
Tap Settings > Wi-Fi on your iPhone. When WiFi is enabled, your phone will search for available WiFi networks. Tap the name of the network you want, enter the password and choose to join. Once youre connected, youll see the WiFi logo appear in the status bar at the top left of your display.
3. Switch WiFi Calling on and off
Tap Settings > Phone > Wi-Fi Calls on your iPhone. Turn the Allow Wi-Fi Calls setting on. If youre connected to WiFi, youll see WiFi Call at the top of your screen and youre ready to call and text over WiFi. It can take up to 24 hours to activate, but you will receive a text message when you are ready to go. Should you wish to turn WiFi Calling off at any point, simply toggle Allow Wi-Fi Calls to the off position.