I have a friend who started when he was six and is now not half bad

Kerioboe - yes, it would be a single Horn. Double Horns are F and Bb (though you always think in F - you don't think a transposition) and a single is one or the other. I've heard of teachers starting pupils on single Bbs because they're so much lighter - I have a full double and the first thing people say when they try it out tends to be "It's so heavy!" They also do Horns that are just simply smaller - I never stop taking the mick out of my friend who is about double my size and yet still has a tiny, almost child-size Horn! All of these may be viable options - he
could simply learn the trumpet before, but I think if his dream is to play the Horn it might just be better to let him start on it (the weight of the instrument seems to be the main problem and that could be solved with a smaller instrument).
When you say North London, whereabouts are we talking? I know of a few places (being in North London myself) but obviously there could be huge amounts of travel involved between them! If you can let me know more specifically I may be able to help