to join the NICEIC DI you need 17th soon to be 18th, a testing qualifcation of some sort, and then pass there multi choice exam on you computer at home with your books next to you - its in lieu of the level 3 course, buy a few books, a multi tester and some insurance then show a piece of work you done and you are in... subject to the cheque not bouncing.
in real terms you need experience, any muppet can join one of these schemes but you need to know what you are doing in order to not just be safe but not endanger the public when working. given the number of people working in the domestic sector it really is getting cut throat and when I was up in London recently visiting another electrician i know I was amazed by the number of people competing for work housebashing up there. experience will help you provide more than the basic service and time gives you the opportunity to learn additional skills and go on other free manufacturer courses.