The question concerns UK, not American, copyright law.

"Fair use" allows small quotes for purposes of review and in academic papers, for example; I don't think there's a hard-and-fast rule. It depends on the circumstances. I don't know about works of fiction. Any substantial quote should in any case always be cleared with the copyright owner, as a matter of simple courtesy quite apart from anything else.

Without precise details of what you want to do, nobody can give you anything but the most general advice, and most people will probably give you wrong or misleading information (see the very large number of previous threads on copyright issues). Consult your publisher, if you have one.