From the advanced guide:

27 percent of those in social class AB found this unacceptable, increasing to 35 percent of the C2s and Ds, and 45 percent of those in social class E.

I'm sure whoever wrote this was just trying to refer to the different groups in a succinct way, but to me this classification system is amusingly reminiscent of a Brave New World's.

These are pretty standard demographic classifications in Britain, used in marketing and such-like:

http://en.wikipedia.org/wiki/NRS_social_grade

In, for example, the media you'll quite regularly come across such things as "Over 50% of our audience are ABC1s".