When it comes to improving performance, the key is to rehearse. Using resources on this website, the reader may be able to practise mock VIVA and OSCEs.

The reader may wish to book himself on a mock-exam course:
The Oculus course in Birmingham.

Alternatively, the reader may wish to improve his speech and delivery:
School of elocution in Birmingham.

The reader may find the following resources useful:
Masters of body language by Dr Gabriel and Nili Raam.
Your speaking voice by Toastmasters International.