Harassment can take many forms, and it doesn't have to involve violence or threats. It includes any deliberate action by your landlord designed to make your life difficult or pressure you into leaving, which amounts to a backdoor eviction. Common examples include repeated unannounced visits, entering the property without permission, verbal abuse or threats, withholding services like heating or hot water, interfering with your mail, and making excessive or unnecessary noise during works.
Your right to quiet enjoyment means your landlord must leave you to live in the property peacefully. If they're deliberately disrupting that, it's harassment.
Harassment is a criminal offence under the Protection from Eviction Act 1977. Your local council's tenancy relations officer can investigate and take action, and the police can be involved in serious cases. You can also take civil action for damages in the county court, and apply for a rent repayment order through the First-tier Tribunal.
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