Complex sentence: The following month, it was confirmed that the band would be playing Open'er Festival in Poland as well as the Reading and Leeds Festivals in the United Kingdom, where they are to headline the BBC Radio 1/ "NME" Stage.
Split into simpler sentences: The following month, it was confirmed that the band would be playing Open'er Festival in Poland as well as the Reading and Leeds Festivals in the United Kingdom. They are to headlined the BBC Radio 1/ "NME" Stage.