Apparently the Avenue gives a "holistic" education. The children do a lot of arts and crafts, gardening etc.
It is not as academic as HHS , but I have a friend who rejected a place at a top boys school to leave her son at the avenue.
The trip to HHS is a pain but worth it. You can take the w5 to archway and then the c11. Alternatively (but I am not sure), doesn't the train from crouch hill go to hampstead Heath train station? It is a very short walk from there. When it is very cold I take a minicab (12 pounds from a similar distance like yours, expensive but not incredibly so).
HHS is a sure bet. Having said that, if your child is not Highgate or Ucs material he won't get in anyway, but the preparation is the best ever.
My son is not a genius (I like being objective, apologies if I don't think my children are the best in the world at everything) but you should see him reading and spelling. Beyond mine and my husband's wildest dreams. And he is only in reception...
And the children are so confident and funny...
Don't discount HHS, it is the best pre-prep out there...