There's lots of great sounding advice about the likely nature of CEM tests on this website. The thing I'm wondering about though is the level of difficulty to be prepared for. Is there a school / group of schools / authority currently using CEM where they are only looking for the top 25-30% of the population (as the Bucks test is required to do?)
Not for state schools, but CEM is also usued in a different guise (generally online) by a number of independent schools, and they also produce other tests - e.g. MidYis which are used by schools for baseline assessments of the the whole ability range. These are generally adaptive in nature so they will probably have a pretty good idea from that of what questions are likely to prove an accurate indication at different points on the spectrum.