IMechE have, for years, excluded anybody not eligible for CEng - that was a policy decision made I suspect shortly after George Stevenson shuffled off. Allied to this, was a grave suspicion of anybody who didn't hold a degree - believing that they belonged in some other "lesser" institution.
About a year ago, there was a sudden reversal, and now they think IEng's are the most wonderful people - so long, one suspects, as there aren't too many cluttering the place up and making it look untidy. All rather odd and distasteful - I prefer to recognise my professional colleagues for their competence, not their tertiary education. Frankly, I belong to the IMechE because they are useful (and they keep giving me awards, and they might stop if I left - the last one came with a cheque!), the attitude there does trouble me - just read the letters page of any issue of "professional engineering" and you'll see what I mean.
G