Hello @Leapfrog1980 ! I am assuming you will not want to move schools after Year 3 and will want to see things through right up to Year 13 at Highgate.
In general, I’m afraid a school exists for its own purposes. Parents and children are there only to make the school look good. The more ‘elite’ the school, the stronger this imperative. As a consequence, schools ‘manage out’ some kids. A ghastly practice, I agree, but all too common. To put it pragmatically, a school has its teaching, behaviour and care model, all within its overall financial budget. If a student needs 'too much' teacher time, if someone’s behaviour is ‘too different’, if a child needs 'too much more' or distinctive nurture, too bad. So, at any school, in the long term all we can do ultimately is trust in the strength of the values, nurture and self-esteem we develop at home.
Notwithstanding that, Highgate has revamped itself overall in recent years, partly through the introduction of girls in all years. From Year 7, it is now the premier destination as a strong, all-round, co-ed school. We know families with boys and girls there and they have been happy with Highgate.
Travel is always awkward in most parts of London and, at the time, was a major criterion for us, too. But I would look at UCS, at least as a back-up. Planning up to Year 13, could you make the travel work?