Halal food as you probably know is that permitted under Islamic law, while Haram is that food which is forbidden. The problem comes with the word "legislation". In UK law there is no specific legislation for Halal food unless it comes under the purview of legislation enacted to protect the rights of animals or that deals with the preparation, keeping and sale of foodstuffs. Simply put, if Halal food accords with UK law then it is acceptable in the UK. A number of UK Moslems however would use the term "legislation" to relate to Islamic religious beliefs regarding behaviour, "Haraam" in this case would relate to anything forbidden such as eating pork, drinking alcohol or extra-marital sex. A friend of mine describes "haraam" as the following:
The rule is that everything is Halal unless explicitly forbidden: as long as there is no authentic statement from the Qur'an or the Sunnah of the Prophet, then it is considered Halal.