I believe that Isle of Man Government needs the approval of the UK Government (ie DfT) to establish a register. As I understand it, an Isle of Man ANO also has to be agreed with DfT. It appears that the section of DfT discussing this issue with the Isle of Man is also responsible for the current consultation exercise.
If the Isle of Man were to adopt FAA standards, aircraft on an Isle of Man register could not be based in the UK under the DfT proposal. While aircraft could be based at Ronaldsway I suspect that space will be somewhat limited.