Complex sentence: Hawking believes a United Kingdom withdrawal from the European Union (Brexit) would damage the UK's contribution to science as modern research needs international collaboration, and that free movement of people in Europe encourages the spread of ideas.
Split into simpler sentences: Hawking believes a United Kingdom withdrawal from the European Union (Brexit) would damage the UK's contribution to science as modern research needs international collaboration. He believes free movement of people in Europe encourages the spread of ideas.