I only messaged Nnaush because I didn't want to majorly awaken the zombie thread, but it's now awake so here goes.
As far as St Martin's and Habs went, I just said that there's a gentleman's agreement between 13+ preps and senior schools, that they don't poach. ie if your son is at a prep, then he wouldn't be allowed to transfer at 11+. Also when you start at a 13+ prep, they ask you to sign a contract stating you agree that you won't shift them at 11+. Though if you're moving to state at that point (I know some parents who've got their DC into QE Barnet, for instance so they've had to go at 11+) then it's more tolerated.
I do know that one parent in my DS's year left after nursery: they lived pretty much on top of Habs so I guess they got in at 4+.
The other thing to bear in mind is that you don't know at 3+4+ etc what your child is really good at, or their character. If you choose a prep, then by the time they hit Y8 then you'll have had a lot of discussion with the head/tutors etc and will know which senior school they are best suited for and will be happiest at, so your sons can do common entrance for the right schools for them. Keeping options open at the early stages is therefore a good thing. Though it's not necessarily easy to get in the preps! I heard there was a 3+ ratio of 4:1 and 4+ ratio of 10:1 applicants:acceptances over all the Northwood boys' schools, though I could be wrong.
St Martin's, St John's and NP/MTP are all pretty good at getting boys into a whole range of senior schools, with a fairly even distribution of scholarships. Even with the new ability of MTP boys to pass into MTS without CE (if a good fit! They aren't expecting all boys to go straight through, just about a third as happens already) the majority of boys will still be doing CE and other exams for other senior schools incl Habs.
Lastly, do try out the journeys from your home to all the schools you're thinking of, in term-time, and make sure you can manage it for years with your DC. We're talking here about schools over a large geographic range and the commute can be quite horrible when you have a tired/fed up/needing a loo child in the car. I am speaking from experience here! Going to a closer to home primary school may help. And remember, there are buses to Habs from Rickmansworth/Northwood/Pinner etc. for the older children.