A hypothesis for you:
IOM registry adopts EASA standards, thereby allowing aircraft to be kept in the UK.
IOM issues licence validations based on ICAO licenses - which would include FAA certificates and ratings. (the main reason people go 'N' reg - IR privileges)
DfT is happy
Owners happy
I believe that DfT probably have more than just airworthiness/maintenance standards in mind. As you may be aware, proposals for EC legislation extending EASA's remit to cover aircraft operations and personnel licensing are expected to be published very soon. It is likely that the implementing rules for flight crew licensing will be based largely on JAR-FCL which, if I understand correctly, currently limits the validation of non JAR-FLC PPLs with instrument rating to a maximum of 12 months. However, whatever EASA decide on validations, their requirements will have to be followed by all EC states including the UK, and presumably the Isle of Man if they wish DfT to permit aircraft on their register to be based in the UK.