Do Master of Research programs require prior research experience?
No, you do not need research experience to get on a UK MRes progamme. Though it will help.

It may vary by institution; this answer is based on the one MRes from my subject area.

The UCL MRes in Energy Demand Studies has the following prerequisites (taken from that linked page):


  
  Good numeracy and literacy and an ability to present ideas clearly
  An appreciation of the importance of behaviour and society in energy demand
  An understanding of or ability to learn basic physics and engineering concepts
  Strong performance in a science, engineering or social sciences discipline
  


The course, in its first half, contains a lot of taught material. This covers, amongst other things, research skills.

Having research skills, or a research track record, will help you with getting onto the more competitive courses, such as this one; and it will improve your chances of getting your study funded too.

One of the selection criteria is (ibid):


  how your academic and professional background meets the demands of this challenging programme


so if you can show a research track record, that will help fulfill that criterion.