Picking up on Sherry_D observations, I would comment as follows:
My DC has recently converted from a state primary to a boarding prep and will transfer to senior school 2013, so the following is based on what I have learnt so far.
Maybe I am confused by why would you want to go from an independent school to a state grammar back to an independent school, all within a couple of years. Surely this would be very disruptive for even the brightess and most able of children.
Senior independents fall into two broad camps, those with 11+ entry and those with 13+. Many of those with 11+ entry are day schools eg Habs, Merchant Taylor, whilst many with 13+ entry are traditional, but not exclusively boarding schools, such as Eton, Winchester, etc. It is not a question of 11+ being easier if the school doesn't have 11+ entry and some of the more traditional schools do not. Also, another point to note is that the grammar will be following the National Curriculumn, but Common Entrance does not follow this entirely and the best way to describe it is similar standard to KS3 but taken at 13 rather than 14.
For some schools, you are already too late for 13+ entry eg Eton, Radley etc for 2013.
Common Entrance is a 2 year course and the exam paper is marked by the school that the child wishes to attend, not by central examiners. Whilst it is a standard test, the subjects on which entry depends, varies from school to school. Also, some senior schools have "preferred" feeder schools, so you need to know which senior school you want your child to go to and then pick the appropriate feeder prep that has a tradition of sending children to that school. That said, it is not an absolute, but during the two year preparation period, the proposed senior school often requests updates from the feeding prep on how each candidate is doing, hence the relationship between feeder and senior schools. Other schools require children to complete pre-tests before they do the Common Entrance exam, which can lead to a child being "dropped" if it is believed that they are not strong enough.
I would comment that you need to be clear in your mind what exactly it is that you want for your child. Competition can be fierce and to change schools after only 2 years without a good reason, such as a house move, may lead to a school looking unfavourably on your child.