I doubt very much that your child will be called for an interview for that intake. Most people register their children within weeks of birth for Thomas's. Thomas's see around 250 children for 80 places and have a very strong sibling policy. (This September's intake has 50 siblings, therefore only 30 places were available to non-siblings)
From my personal experience, Eaton House seems to have the most movement on its lists. We called in May 2007 to put our then 4wk-old-son down for the Sept 2011 entry and were told the main list was full so we didn't bother putting him on the waiting list. Then in April 2010, we began to get nervous that we had no back-up plan (having only put him down for Thomas's and Broomwood Hall, two competitive entry schools) We were advised by the headteacher of our nursery that it was well worth registering him for Eaton House, even though the waiting list had now been growing for 3yrs. We registered him, parted with the obligatory £50, then got a call in late October 2010 offering him a place. The only major downside with Eaton House is the size of the deposit required. Thomas's and Broomwood each take £2K as a deposit, Eaton House ask for a term's fees, somewhere around £4K. I know some people who panicked and paid £4K to secure a place just before going for the Thomas's interview and getting a place there.
A friend, whose child is starting at EH this autumn, has told me that there are still lots of places available for this September's entry.
Good luck with it all, it's daunting at first, as every school has its own system, but don't get too worried, especially if you're only staying a few years. In that context, every school around here, incl Belleville and Honeywell, will fit your needs perfectly. x