The type of insurance you will need is Contractors All-Risk Insurance (also know as Contractors Works Insurance) it is pretty easy to get. You can read more about it here
http://www.designingbuildings.co.uk/wik ... _insurance
Also, it is important to ensure your builder has Employers Liability Insurance and Public Liability Insurance before they start work (make sure you see the policy). If you builder is hiring subcontractors (ie the builder pays them) make sure the subcontractors have Public Liability Insurance and check if they need to be named on the main builders insurance. For subcontractors you hire directly they will need Public Liability Insurance and Employers if they have employees.