There's nothing to stop you getting some text books from an ex-student and starting to brush up but you can't enrol on a course until you have your PPL. There are often various course notes for sale on fleabay but don't pay too much for them as you'll have your own set when you enrol on a course plus you'll be giving them house room rather than the files cluttering up someone else's house! If you're near Norfolk, you can have some of mine!
You have three years in order to get an IR after you've passed ATPL exams. If you don't, and later want an IR, as far as I know, you just have to sit the IR related subjects again (about half of them!)
However, if you go for CPL exams, please be aware than CPL exams plus IR exams will not get you an ATPL (with required experience) anymore.
The ATPLs are not much harder than CPL; just a bit more volume. There are certain areas of the ATPL syllabus which are not covered at CPL. In fact, none of the subjects is hard, just vast! And boring!
Cheers
Whirls