Try asking the IOM3 & other accrediting institutions why the courses you are interested in are not accredited and if there is a possibility that they could be accredited in the near future.
The courses may not cover material that such institutions consider vital for professionals engaged in that field and which would be required for membership of the institutions.
If you were to study one of those courses you may need to study the missing subjects elsewhere prior to being eligible to for membership of the institutions.
Regarding employment prospects, it's safer to study an accredited course. In times of economic downturns or gluts of professionals in particular fields employers can be very discriminating. Anyone without an accredited qualification can more easily have their application disregarded.
It's not just job prospects in the UK that you need to consider; materials science/engineering is an international profession. Larger companies track the performance of universities and the courses they offer and they know which universities offer courses that are of most interest to them.
Companies also prefer to employ people with accredited qualifications so as to minimize any legal issues should there be a problem arising from their products or services and the personnel responsible for those products or services did not have an accredited qualification.