I don't have any experience of Elstree, but I gained my PPL with Cabair a couple of years ago and found them excellent. I went through the same headaches everyone does when choosing a training organisation. At the time Cabair were marginally cheaper than some of the other London organisations, but it's very difficult sometimes as you're not always comparing apples with apples - what's included in one organisations packages might not be in someone elses. Judging the value of these differences is not always easy. If you're interested in Denham - drop me an e-mail. I recently wrote quite an extensive e-mail to someone enquiring about flying training at Denham and I'll be happy to forward it on.
Whatever you decide to do and whoever you choose - the very best of luck with it.
Suction