When you press play on a programme, we serve the lowest bit rate (quality) first. In the following seconds, we'll amend the quality based on your internet connection speed.
That way, you get the best quality version possible without the need to change any settings yourself, and it allows us to start playback as quickly as possible.
While it's not that noticeable on smaller screen devices, it can be more apparent on larger screens, so we're looking into ways to improve this experience. Meanwhile, rewinding to the beginning of the programme once the video quality has improved should allow you to watch the programme in full without any change in quality.