Chackal - we're on the case and will do what we can to sort this out for you.
When the NPPL was first created, the not unreasonable view taken was that, because it required substantially less training and testing time than the 'normal' JAR-FCL PPL, all flight training had to be conducted within airspace in which the licence itself could later be used.
At that time, the NPPL (SSEA) could only be used in UK airspace; ergo the training was similarly restricted.
However, the IoM very quickly agreed that the NPPL (SSEA) could be used in its airspace. However, no-one at that time thought about the associated training.
I see no reason at all why you should not be able to train for your NPPL (SSEA) on the IoM and will be asking my colleagues to seek confirmation from the relevant authorities as soon as they can.
We want you to fly!!