Firstly, libel, it's always with us, but elections are that special time when candidates lay into each other with abandon and occasionally say something defamatory. Remember it is no defence to say you are simply reporting what someone else said (but see below). Anyone who repeats a libel is potentially liable for it and a defamed candidate may decide to sue the relatively wealthy media outlet that has repeated the libel rather than the relatively poor opponent who originated it. Beware accusations of racism, fascism and plain old lying.
However, if you are reporting remarks made at a public meeting, or press conference, then you have a defence of qualified privilege, so long as you are reporting fairly, accurately, on a matter of public interest and without malice. Don't get overly worked up about malice - the malice of the speaker does not 'infect' your report of their speech and has never yet destroyed a defence of qualified privilege mounted by a media organisation.
False statement about election candidates. Section 106 of the Representation of the People Act 1983 makes it a criminal offence "to make or publish a false statement of fact about the personal character or conduct of an election candidate, if the purpose of publishing the false statement is to affect how many votes he/she will get."
The false statement must be statement of fact, not opinion. It is a defence to show at the time of publication you had reasonable grounds for believing the statement to be true. A journalist who published false claims that a candidate was homosexual was fined £250 in 1997. Note that it is no longer defamatory to say that someone is gay (unless it implies they are dishonest by concealing their true sexuality) but it would contravene this law. The reason being that if the voters included those whose religious beliefs cause them to hold anti-gay views, then such a statement could affect turnout for the candidate.
The 1983 Act also makes it an offence to publish a false claim that a candidate has withdrawn from the election if you know the claim is false and it is being made to promote the election of another candidate.
Impartiality of broadcasters. The Ofcom code and BBC Editorial Guidelines have detailed guidance on achieving impartiality. Several radio stations have been fined by Ofcom after presenters declared political allegiance on air.
Exit polls. Section 66A of the 1983 Act makes it an offence to publish the results of an exit poll before polling has finished. The reason being if the exit poll reveals a runaway winner it may discourage people from voting an thwart the democratic process. It is also an offence to publish a prediction of an election result if it is based on such a poll.
Election counts. Admission to the count is the responsibility of the returning officer. There is no national media policy, so best make contact early to make sure of arrangements for the night.