Hi Andy,
I am currently going through the process to become a Chartered Environmentalist, and it is tough (tougher than CMIOSH, and I have an environmental degree!)
In the UK, you use IEMA for accreditation. The system is based on 12 points. Each year of academic study is 2 points - so a degree is worth 6 points. This, however, has to have an environmental slant (so my degree in Environmental Geology gets me the points). In addition, each year working with environmental responsibilities, full time, gains you an extra point. However, if you are EHS, and E is only half of your role, then it takes two years to get a point. For myself, I have 10 years experience in EHS, so this is worth 5 points, and 3 years degree, worth 6 points, so I have to wait another 2 years to gain my last point.
In addition, you already have to be AIEMA and MIEAM, however you can apply for MIEMA at the same time as CEnv. You have to put a portfolio and application letter together, and then have a peer review interview, usually over the phone (to cut down on carbon...).
Hope this helps - IEMA do great leaflets on the subject, so that would be your starting point.
Good Luck!
Ros