If you have a CPL/ATPL and let your medical lapse, you do NOT revert to a PPL unless you specifically have a separate PPL (there's a tick box on the application form). Such PPL privileges stem from the other licence, and they lapse with that licence. A CPL requires a Class 1 medical.
That said, you would only want the ATP if you are going to fly heavy machinery, and on top of the (14) exams and hours, you also need multi crew training and a type rating on a suitable (large) helicopter which, realistically, can only be done within a company. Having done the ATP exams, you must obtain the IR within 36 months.
BTW, Bristol and Cranfield are not the only game in town for distance learning
Phil