Having read the full transcript (200 pages) of the ruling from the Electoral Court on the Tower Hamlets (“Mayor” Lutfur Rahman) affair I am now well versed with some aspects of the representation of the People Act, 1983, which covers such things.
It seems from the Act (s.106) that it is only an offence to make false statements about a candidate:
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106. False statements as to candidates.
(1) A person who, or any director of any body or association corporate which—
(a) before or during an election,
(b) for the purpose of affecting the return of any candidate at the election,
makes or publishes any false statement of fact in relation to the candidate’s personal character or conduct shall be guilty of an illegal practice, unless he can show that he had reasonable grounds for believing, and did believe, that statement to be true.
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In fact it was found that Mr Rahman had indeed done so about one of his opponents, the official Labour Candidate Mr John Biggs.
Although I have only glanced through the Act itself (the law relevant to the judgement was explained in detail in that text) it seems there is no bar on candidates telling packs of lies about anything else.