Hi Bob,
You should talk to some of your aircrew chums about this. When I was doing DL via PPSC I used to bump into a lot of RAF chaps who all used to share the same theory books, I think perhaps there was one set per camp. Many of these guys didnt actually bother doing a DL course and just learnt the questions did a consolidation course and jumped straight into the exams. I dont know whether you could do this now under JAR. Basically I dont think there are any short cuts to getting your CPL and FI rating.
Currently an FI earns £12 - £15k a year, this isn't a salary so if you dont fly you dont get paid. You may earn a little more with commercial instruction and if you aspire to be a PPL/IMC/170a examiner you can actually make a living, providing you dont live in Severn Oaks with a £350k mortgage, 6 kids and a Lambourghini.
Presently instructor jobs are scarce but by the time you finish your sentance with the RAF things may have changed
Hope this helps
cheers
MJR