101, meaning basic, when it comes to knowledge (as in Photography 101 or Cooking 101), is not used much in Britain. I had to look it up when I came across it recently.
But where could I look it up? It's easy enough to look it up in an online dictionary as you can type in the digits 1-0-1, but what about the big dictionaries I have on my bookshelf? The answer is "it depends on the dictionary". The single-volume ODE has about ten 'words' made up of digits (including 9/11, 20/20, 24/7 but not including 101) at the very end of the dictionary - after the Zs. In Collins I will find the entry 9/11 (written first as 'nine-eleven') in the Ns; it's the entry immediately after nine-days wonder. 24/7 (written 'twenty-four-seven' first) is the entry immediately after twenty and immediately before twenty-one (meaning the game of pontoon). Collins doesn't contain 101 either.
101 is a relatively new slang term but, interestingly, it is in the OED (at the very end, after the Zs, but easy to find in the online version), with a citation from 1929. The two earliest citations in the OED (1920s and 1930s) use 101 in its original sense - where it meant a beginners' or introductory course at a college - as in the citation from 1936 "Art 101 provides a historical introduction to art". By the 1970s it was being tagged on to other words, not just college courses. A 1972 citation in the OED begins "Since this is a series that emphasizes Social Relevance 101 — a basic course on TV these days — ...". 101 is described as an adjective in the Dictionary (it's a postpositive one), and as a postmodifier. The Dictionary also says that it is chiefly humorous when used in the broad sense.