The old self improver route used to be such you would obtain your PPL then study for and pass the CPL exams (later on they allowed the ATPLs to be done without having done the CPLs first) then obtain a BCPL at around 200 hours. One would then obtain a CPL at 700 hours and an ATPL 1500 hours. Once you had the CPL at 700 hours you could then attend a non-approved IR course and train as necessary for the IR. The high hours required generally meant people obtained AFI ratings to work as a flight instructor (upgraded to a QFI after a few different requirements being met).
Today the modular route has taken the place of the self improver route but the required hours have dropped and the BCPL scrapped.
Though I might be wrong about the old self improver route I described above.