Any damage caused is criminal damage. That should be reported to the police.
Do you share any living space with the 'occupier'.? I.e lounge, kitchen, bathroom. If not then the occupier may have basic rights and you will have to serve the proper notice, likely to be two months under the AST you entered into. When was the tenancy granted and for how long?
Break clause has nothing to do with non payment. It's a clause allowing early termination.
The 21 day clause is probably the forfeiture clause but this is fairly toothless in residential Tenancies. A small payment towards rent could defeat the provision.
This is a simple question and answer service. You have a difficult situation. It will depend upon the living arrangements as to whether the occupier is excluded or has basic rights. The tenancy agreement which you and the tenant completed has to be considered in light of the status of the tenant. If the tenant has basic rights then proper notice must be served and you will need a possession order from the court.
Non payment of Rent is a ground to apply for possession and depending on the type of notice served there is a set notice period, which can be as little as two weeks. Damage to property is another ground.
I would advise going to a local firm which has a property litigator. Have a ring round, sometimes you can get an initial meeting free if charge. Ensure to have at hand details of arrears, any written demands, and written correspondence between you and the occupier, details of intimidating behaviour (what when), details of any damage to property, what / when. The litigator can assess the appropriate notice and serve.