I'm looking for a book I used to love when I was a teenager for my cousin. I think it would be in the 2000s now in the UK.
It was about this community that lived underground and another that lived in the city above. A young boy wanted to come to the surface as he dreamed of dragons and there was a girl in the castle above whose mother was of status.
When they both dreamed they turned into dragons. There was another boy who pretended to be a servant who was actually the "bad guy" who also became a dragon when he dreamed and they ended up fighting as dragons and killed him. I think one of the underground characters was called Scrub?