HODGY,
NATS have no idea how to select a good controller, and as such they change their requirements regulary. During my time tehy have had requirements ranging from O levels to a degree and anything between. Look regularly at the requirements:
http://www.nats.co.uk/recruitment/index.html
If you had any studies for your ops position then that could well qualify you for the "post-GCSE" studies (note it doesn't say anything about passes!). As an Ops person, you are also very experienced in the industry and would probably be too good to get through personnel's careful scrutiny
Give an application a go. See what happens!