Yes, unfortunately Google does pay attention to the "in". More so than it does to punctuation in between the words "dentist" and "London" - so what I tend to do in these situations is have one sentence end with "dentist" and the next begin with London"
e.g. "...you always want to be sure you have the best dentist. London is full of options..."
Or if I can manage it, a comma in between the words is almost better, so I would be saying something like "...if you are looking for a trustworthy dentist, London has more than enough options.". In fact, that sentence reads OK without the comma. As for your H1, if you make it "How To Find A Dentist: London" then it looks more natural than without the colon, but still works.