If your institute is running an written exam centre you should have an exam co-ordinator (often the Education Secretary) to manage the exam centre, and be the key contact for the CII Customer Service (CS) exams team based in London.
This individual will have a number of responsibilities including the sourcing of a suitable venue.
An overview of a Education Secretary's typical responsibilities can be viewed here.
The selection of a suitable venue should be done in conjunction with the CII examination team who will advise you some set criteria, this will include:
Many institutes use venues that already cater for examinations being run by other local organisations or institutions such as professional bodies or universities, as these will invariably be well suited to your requirements in terms of equipment and space.