A tweet from HTC's U.K. division (see, others at HTC besides HTC Americas president Jason Mackenzie know how to use Twitter) says that the HTC One has started to receive the OTA update that brings Android 4.3 and Sense 5.5 to the phone, in the region. According to the tweet, if you live in the U.K., you should take your HTC One, and go to Settings > About > Software updates. This might not be necessary if the phone is already showing the notification that it received the update OTA.
The update brings software version 3.62.401.1 to your handset and adds enhancements to Daydream while also adding OpenGL ES 3.0 support. Sense 5.5 brings some new features to BlinkFeed, adding Instagram and Google+ feeds among other changes. The Gallery is improved in Sense 5.5, which also brings a new UI for Video Highlights.
If you live in the U.K. with your HTC One and have yet to receive the update, don't pout. These updates rollout in waves, so be patient. Your update will come.