After completing 40 classes, you are eligible to do your first grading (6th kyu).
Aikido grades are non-competitive. They are a means to advance your level of training and challenge yourself. The grades are 6th kyu, 5th, 4th, 3rd, 2nd and 1st kyu (all white belt), shodan (1st Dan - black belt), 2nd Dan, and so on.
You must become a member of the British Aikido Federation (BAF) before you can take a grading.