Come across a situation with a friend of mine which I'm not sure is actually illegal or just really bad practice, they live in university accommodation and while all residents are out one of the staff/cleaners arranged fridge magnets into offensive messages about the residents, due to prior tension this triggered an altercation between them, has the cleaner broken a law or is this just bad practice?
Defamatory and offensive are not the same
“He is a child molester” is defamatory and illegal (unless he is, of course).
“She is a two-faced f___ing b___h with the morals of an alley cat and the integrity of a politician” is offensive and legal.
Neither is a crime.
Being "illegal" does not mean that it is a crime. It is possibly illegal, but not necessarily a crime. It could be breach of contract or defamation, neither of which are legal. The former case might not even involve the courts, and in the latter case would involve a lawsuit but not prosecution. It might be a crime under law outlawing hate speech, Public Order Act 1986 §18 or especially §4a which forbids a person
with intent to cause a person harassment, alarm or distress... displays any writing, sign or other visible representation which is threatening, abusive or insulting, thereby causing that or another person harassment, alarm or distress.
The content of the message would matter.