1. Firstly most FTL sections of company manuals will specify that 'positioning' is moving crew by surface or air transport at the behest of the company. This means that the company are obliged to lay on transport for you. If it suits all parties for you to 'self position' then thats fine.
2. To clarify the point with regards to on-duty. Longarm is quite correct however there are some exceptions.
Assume you lived in MAN but were based in BHX. If your roster showed a 1300 report at BHX for transport to MAN arriving at 1500 and you left your home, self positioning at 1430 to report for 1500 at MAN, there is no reason why your report time can not be 1430 for FDP purposes. I would suggest for financial reason however that your duty pay is paid from 1300!!