You can read books to gain some of the knowledge you would get from a course, the book that immediately springs to mind is the RYA Navigation Handbook by Tim Bartlett as a first step.
However, there are obvious limitations. You will not be able to test yourself effectively using books to learn, and if you don't understand a topic, there will not be an instructor to explain it to you! It is also common for Theory courses to deviate slightly from and expand the set syllabus, to respond appropriately to questions or particular class experiences, so you can gain knowledge from the other students, not just the instructor.
The best way of avoiding 20 weeks of night school would be to take the course as a 5 day intensive. From RibNet, Doug Innes at Stormforce Coaching runs them, Tim Griffin at Griffin Marine, Paul Glatzel at Powerboat Training Uk, and I don't teach them but look after the admin side of things at Haslar Sea School (apologies if I've missed anyone off this list - it's just a start).
If you give any of the above a call/drop them an e-mail I'm sure they'll be able to help you out with a best course of action that doesn't involve Night School. At Haslar Sea School we run a 2 day navigation course which is pretty good for Powerboaters who don't wish to do a 5 day course geared towards Yachties and Motor Cruisers, and I think Stormforce do something similar.
Hope that helps.