As usual, we suggest (as does HTC) that you secure a Wi-Fi connection before downloading the update. After all, you don't want to waste your precious data on an update. Also, make sure that battery is at least 50% charged. No telling what might happen if your phone turns off in the middle of the process, before the entire update is downloaded onto your device.
Since the update is being sent out in a staggered fashion, keep in mind that you might not receive it immediately. Be patient, and keep a stiff upper lip (this is for the European market after all, which includes Londoners), and the update will eventually find your HTC One (M8)