If anyone has time to spare, compare the requirements of each of the list of chartered professionals that Moshe has posted with the UK-SPEC IEng standard. I think you will find that the evidence supports my hypothesis that IEng is broadly equivalent to a number of other Chartered Professionals.
Unfortunately the proliferation of chartered titles has created a mindset that only 'chartered' equals professional so 'incorporated' is always going to lack desirability. The proposed changes to UKSpec, if implemented, will not improve the marketability IEng and are likely to increase the attractiveness of alternative technologist designations.
