In general, political campaign material in the UK is not formally regulated, and it is a matter for the press and public to decide on the basis of such material whether they consider it reasonable and accurate. While the Advertising Standards Authority (ASA) is the UK's independent regulator for advertising across all media, political adverts are not within its remit. The ASA suggests that anyone with concerns about a political advert could contact the party responsible, and exercise their democratic right to tell them what they think. The free press also has an important role in holding politicians and campaigners to account.
Of course, the wider law does generally apply to political campaign material, including the law of defamation and public order offences. Electoral law also makes it a criminal offence to publish false statements about a candidate: the courts do enforce this legislation, as illustrated during the April 2015 election court ruling which disqualified the mayor of Tower Hamlets for a litany of illegal practices. Electoral law also requires for parties and other campaigners to include an 'imprint' on their campaign material, identifying its source, to ensure transparency and accountability.