Albert Venn Dicey is considered to be one of the classic authorities on (British) constitutional law. Here's what he had to say about sedition:
Every person commits a misdemeanour who publishes (orally or otherwise) any words or any document with a seditious intention. Now a seditious intention means an intention to bring into hatred or contempt, or to excite disaffection against the King or the government and constitution of the United Kingdom as by law established, or either House of Parliament, or the administration of justice, or to excite British subjects to attempt otherwise than by lawful means the alteration of any matter in Church or State by law established, or to promote feelings of illwill and hostility between different classes. And if the matter published is contained in a written or printed document the publisher is guilty of publishing a seditious libel. The law, it is true, permits the publication of statements meant only to show that the Crown has been misled, or that the government has committed errors, or to point out defects in the government or the constitution with a view to their legal remedy, or with a view to recommend alterations in Church or State by legal means, and, in short, sanctions criticism on public affairs which is bona fide intended to recommend the reform of existing institutions by legal methods. But any one will see at once that the legal definition of a seditious libel might easily be so used as to check a great deal of what is ordinarily considered allowable discussion, and would if rigidly enforced be inconsistent with prevailing forms of political agitation.
(A V Dicey, An Introduction to the Study of the Law of the Constitution (1885), ch VI "The Right to Freedom of Discussion")
Neither our sedition laws or our prevailing forms of political agitation have changed significantly since Dicey's day, and his criticism still stands. Almost any criticism of the government can be said to "excite disaffection", and almost any speech or article on race relations, employment relations, or welfare can be said to "excite... hostility or ill will between different classes of persons". Both of these constitute sedition, which means that much of contemporary political discourse is technically illegal. While the law is not enforced, that is not a sufficient proection in a democracy. It should be repealed.