My understanding of CAP371 is this. You can operate your FDP as defined by sectors/start time and the FDP finishes at on chocks at airport C. The company can take any route back to A they like. As far as I can tell, they could position you to the other side of the world via LHR T5 and that would still be legal. Immoral, but legal.
Remember it is the Flying Duty period that finishes at C, not the total duty period. All that is then required is a rest period equal to the total duty before commencing another duty.
My company will provide HOTAC at home base if the total duty exceeds 16 hours. Your company has a duty of care to you. You may not be fit to drive home after an extended duty. Remind them of this.