I just wanted to add a little more to this because I think that the school you are talking about probably differs a little from general independent education.
Because of the demographics of the area I think that there are a lot less of the scrimping and saving parents at the school than there might have been had it been located elsewhere.
Firstly, there is a huge jewish community in the area nd over the last 10-15 years many jewish families who were themselves educated privately have opted out of the independent system for their own children due to the growth in jewish state schools. These schools offer superb levels of education and the need for them to send the children privately has simply diminished. Rosh Pinah and Mathilda Marks are both within 2 miles of Grimsdell and rank 1 & 2 in barnet tese families are probably ones who could afford private education but have chosen not to and a more of a "normal" sort of family rather than very wealthy.
There are also very good local state schools including Courtlands and Frith Manor which again attract parents who, if they weren't so good would probably go private.
Add to this the other 3 private schools local to Grimsdell which are Goodwyns, St Martins and Holland house. They have nowhere near the facilities that Grimsdell offers but they're also over £1000 a term cheaper meaning that again, there are less well off parents making that those schools their first option.
Add onto that the parents who then send their children to places like Habs and North London, again, just up the road and the pool of people opting for a specific school becomes smaller.
Again, I think that Grimsdell is great, but because of the other good local options, not to mention very expensive housing in the area, a 3 bed semi in parts of Mill Hill can go for over £600k, there may be more wealthy families than in other independent schools. That's not to say they won't be lovely, I know lots of people there who are great but it's not a standard demographic