You cant do single pilot ops without 700 hours (JAA). You can do multipilot ops with less than 700 hours.
So to get the required hours for an air taxi outfit you have 3 choices:
1. Pay for the extra 350 hours required or buy an aeroplane.
2. Work for an airline to get the 350 hours, leave your big shiny jet and then do your single pilot ops.
3. Get an FI rating.
I guess that explains why, with current market conditions, so many people are going the FI route.
When you have the required instructing hours you could always instruct on a twin, night instruct, IMC, CPL/IR...
SEP hours are equally valid for reaching the magical 700 and all of the instructors I know enjoy the job for the first 1000 hours or so.