You just email them saying you would like some work experience and are interested in volunteering for a hospital. And that if they could provide guidance on how to go about with this then that would be great.
They will then send you an application form which you complete either by email or post and send back.
They will then invite you for interview and you will need to take NI, Proof of Residency and passport.
This can all take a while though because there are so many people wanting to volunteer for the NHS.