For me, I chose my department with the NHS (A&E) and I emailed one of the consulting doctors for work experience and my reasons for it and she emailed me back a form to fill in (this was a little bit unconventional as I had been in A&E the day before so I didn't give her a CV or anything)
Most work experience placements will require you to give them a CV and a cover letter (very simple to write) and usually you would (according to what my friends have done) email a specific department and the head of the dept.
With NHS, most of it is shadowing consultants/junior docs, there were a number of things I couldn't do as I didn't have the experience or skills but I did get to do things like tests with blood samples and ebola training.
Whatever the limitations are for performing tasks with your work exp though, it's a great opportunity to ask people questions and the reasons why they're doing their job and other questions that you could ask for your UCAS personal statement/interviews.