I havn't any idea what GMPA was actually doing in the IoM, however police role equipped machines [usually oldies from the mainland] regularly get hired either for events, operations or just to keep the nominated crews trained up.
As for GMPA its just one of many kept in trim by a range of operators who keep them role equipped 'just in case'.
Main users are the railway police but all the forces without direct access to air support [ie Kent, Lincs, Cumbria, IoM] might be seen using them for time to time.