Which is the best literary agency that handles romance for the YA audience in the UK?

Try buying yourself a copy of "Writer's Handbook 2011" or "Writer's Market 2011". You can find them in Waterstones and WHSmith. It's got plenty of legit agents. If I were you I'd stick to approved books like those rather than searching on the Internet. The books may cost money, but at least you know you won't get scammed.