Okay, had a chance to think about it some.
Looking at the numbers you're putting out, my initial inclination is to put the rarity of adepts and mages at about one order of magnitude higher than the initial numbers: so 1 mage to 10 adepts to 1,000 sensitives to 10,000 normals.
However, these numbers for adepts and mages represent potential rather than the actual fraction of the population. For mages, my current thoughts are that out of every 10, somewhere between 8 and 9 die prematurely (mostly at the hands of other mages), quit, or live quiet lives (which makes them functionally indistinguishable from non-mages unless you go digging into it).
So going with the population of London as 8 million (rounding up) that gives roughly 80 practising, active, and involved mages and 8,000 adepts. The adept number seems too high at first glance, but that does include ones who have weak/useless abilities or who never particularly develop their powers. Haven't decided on those ratios, as most of the series so far has focused on mages rather than adepts (though the new book I'm planning at the moment is going to deal with them a bit more).
With regard to the ratio of the magic families, I was thinking somewhere in the neighborhood of 55%-60% elemental, 30% living, and either 10% or 15% universal.