Well, I am not telling anyone to be a grass and I believe these laws to be draconian.If I see any racist abuse on here I'll bring it to their attention. What about general abuse?
Generally the Malicious Communications Act, but there is a high bar in the sense that the prosecutor has to prove the person sending it intended it to cause distress or anxiety to a particular person. The message can fall under this offence if it is either: A message that is indecent or grossly offensive; A threat; or Information that is false and is known or believed to be false by the sender.
The Communications Act offence of sending grossly offensive or "indecent, obscene or menacing messages" has a lower bar, in the sense that the person sending the message only has to "have awareness or recognition that sending the message may create fear or apprehension in any reasonable member of the public who reads or sees " (i.e they don't have to intend the message to cause fear and apprehension).
A further offence under the CA is an "intention to cause annoyance, inconvenience or needless anxiety" by sending a false message. However, that requires the sender to intend the other person annoyance, inconvience or needless anxiety.