GtE is correct, although it's 15 exams (IR HPL credits for the CPL equivilent). Technically there's nothing whatsoever to stop you flying as a first officer having done the CPL & IR exams as opposed to the ATPLs. The only difference is that when you get to 1500hrs (and other requirements) you cannot take the skills test and have your ATPL issued because you don't have the correct theory credit. No ATPL = No Command, hence you would only ever be a F/O.