You may want to be asking not only where the HE families are, but also where good communication and organisation is happening. It's so much easier to make friends where there are things going on. Even if you and your kids don't like groups and are just after individual friendships, you'll find that more rapidly in an area where there's an excellent network.
What ages are your kids? In some areas the HE families seem to have got their act together with respect to teen activities, whereas in others there is little in the way of organised stuff beyond primary level despite there being plenty of HE teens around. It may be slightly trickier to make friends in the latter areas, as most of the teens are already occupied doing things informally with small groups of HE friends and/or participating in non-HE activities.
Someone on an HE list wisely observed that to make new friends, adolescents often need more structure and proactive help from parents than little kids do. They start getting self-conscious and more particular about their friendships, and you can no longer haul them out to meet people over their objections. Until they know each other fairly well, they would rather have an active purpose to the meet-up rather than being chucked together and told to play!
Do you want somewhere urban or not? Though we have a car, I love being in a city because of the freedom it gives my teen to roam around alone, see her friends and do more after-school activities without needing the mum-taxi. I expect this will also give her more options when she wants to start working, volunteering or maybe going to college.