We moved to the UK when eldest was about to enter Year 7. We were late on everything, so the only place we could find was in a traditional prep school.
It worked out really well. He went into a class of really nice boys who were very sporty etc. He got the individual attention needed to enable him to catch up with the curriculum parts that were lacking, eg French.
It was also really nice for him to be at the top of the school for a couple of years, and was able to be a prefect, get a decent part in the school play, etc.
He joined the senior school we had originally targeted in Year 9 and has really flourished. The prep school curriculum covers a lot of Year 9 work so it was an easy transition academically.
As for Years 7 and 8 being add on extras - no, they are real years. They are two precious years in a young person's life. I really think a 13+ transition is good for boys, but 11+ is better for girls. Girls are really ready to move on somewhere in the middle of Year 6, but boys are happy to stay in the smaller environment.
Poppy,
I have a fairly pragmatic approach, so I would try to find out about the admissions process for both the prep school and senior school at 11+, and again the senior school at 13+. You basically have to have a senior school goal, and then do what it takes to meet it. If the chosen senior school does not have a CE intake, then you have your answer. If it is a choice, then seriously consider the prep school first route. Your DS may value the extra nurturing and support that a prep school gives given than he is moving from overseas.