You are correct, chary, the test study guide was for me an essential and I really think it helped me pass my theory with flying colors.
Yes, you don't actually have to have learnt the entire Highway Code by any means. But it's a good idea to own it and to have at least given it a good going-through, and familiarize oneself with road markings, signage and rules of the road that can often be similar-yet-different to the US ones.