It's a bit more limited than that: during election campaigns (which last a few weeks in the UK) it's illegal for anyone to lie about other candidates' personal character with the intention of affecting the vote.

e.g. s106 Representation of the People Act 1983 ("False statements as to candidates.") http://www.legislation.gov.uk/ukpga/1983/2/section/106

Everything else is fair game and "personal character" is narrowly construed — it's about "the man beneath the politician" as one court put it.

There's also no legal bar to lying about your intentions in your manifesto, despite a few attempts to persuade the courts to create one.

This from the High Court of England and Wales — "Can a promise of this kind give rise to an enforceable legitimate expectation? Even if we had accepted that the relevant ministerial statements had the effect of a promise to hold a referendum in respect of the Lisbon Treaty, such a promise would not in our view give rise to a legitimate expectation enforceable in public law, such that the courts could intervene to prevent the expectation being defeated by a change of mind concerning the holding of a referendum. The subject-matter, nature and context of a promise of this kind place it in the realm of politics, not of the courts, and the question whether the government should be held to such a promise is a political rather than a legal matter." (Wheeler v PM, a case about whether the PM was legally obliged by a manifesto commitment to hold a referendum on an EU treaty http://www.bailii.org/cgi-bin/markup.cgi?doc=/ew/cases/EWHC/...)

(IANAL).