Complex sentence: Most of the PictureBox films will be available in HD and many will not yet have been seen on free-to-air television in the UK, it marks the PictureBox's first UK platform to present films in HD.
Split into simpler sentences: Most of the PictureBox films will be available in HD and many will not yet have been seen on free-to-air television in the UK. It marks the PictureBox's first UK platform to present films in HD.