Consider thinking about why you want to be a final signatory first. The quickest way might not be the most fulfilling way! Sitting in front of a computer looking at job bags all day might not be what you actually want to be doing. You might want to go into medical affairs work on some interesting projects like congresses, advisory boards, launch plans etc and maybe spend 10-20% of your time learning about review and approval to become a signatory. 

Becoming an ABPI signatory takes time. Similar to validating prescriptions when you sign something off you are taking responsibility that you are confident that it meets the requirements of the regualtions. It is important that you are able to grow your knoweldge and build your experience before you become a final signatory. 

There may be more direct routes to becoming a final signatory eg working for a 3rd party company that is an outsourced signatory specialist. These companies usually hire candidates to eventually become signatorys, however it may still take a few years.