After receiving significant community feedback, Microsoft announced back in May that it would work on adding "double tap to wake" for compatible Windows 10 Mobile devices. Since then, the feature has rolled out via firmware update to those in Russia, Sweden and India. In early July, Australia also received the updated firmware and now it looks like though those in the UK will finally have it as well.
However, double tap to wake is not the sole improvement included in the update, as the official changelog mentions a number of improvements to Wi-Fi, Bluetooth and cellular connectivity, along with a number of refinements to the camera and video quality.
As with other firmware updates, you may not be able to download the update immediately, as it is most likely a staggered rollout. But, if you absolutely can"t wait and must have it,you can try using the Windows Device Recovery Tool (WDRT) available here. However, updating via WDRT comes at the expense of completely wiping your device, so make sure you perform a back up of your phone before proceeding.
Source: MSPowerUser