We have a Single Checking Administration of Medication (SCAM - I'm the only one who thinks that's funny!) policy. Staff must be approved by the NUM, have been practicing for over a year, be familiar with medications commonly prescribed in the unit, be generally competent and skilled, pass an initial and annual competency, and keep a log bok demonstrating safe practice in medication administration.
Any medication can be single-checked (the only exception is blood products, which must be double checked by Federal law), provided the practitioner is confident in her/his ability. In practice, on my ward, that means that we pretty much all hang heparin infusions and give sub-cut insulin, some of us make up insulin or lignocaine infusions, and none of us make up cardiac infusions (because we give them so rarely).
The initial test and annual competency are created by the senior staff (NUM, ACNs, CNSs and the Clinical Support and Development nurse), and are ward-specific. So on our test there are a lot of questions on mixing insulin, trouble-shooting (eg patient with type 1 diabetes has a low BSL, is due X units of this and Y units of that... and what if they were type 2, insulin-requiring? etc)
One of the key points stressed in the whole SCAM concept is that any nurse should feel completely comfortable asking for a double check or advice with any drug at any time. In fact, staff that the NUM doesn't think will ask for help are less likely to be approved.
Hope this helps