If you want to work on the North Sea, you will need a CPL/IR. Irrespective of the exams you take, you cannot obtain an ATPL without first obtaining the CPL and an IR. This is because to obtain a JAA ATPL you are required to have experience of piloting Multi-Crew aircraft. As mentioned in an earlier post, you only require an ATPL if you are the Captain of a Multi-Crew aircraft.
It is important that you understand this distinction. You do not get an IR through completing the ATPL exams; you obtain an ATPL through completing the ATPL exams, obtaining an IR, gaining experience in a twin-crew aircraft, plus all the other hours and experience requirements as laid down in JAR-FCL 2.