No matter what your landlord says or what your tenancy agreement states, the only lawful way to evict you is through a court-issued possession order. If your landlord takes any action to force you out without going through the courts, they're committing a criminal offence.
Common forms of illegal eviction include changing the locks while you're out, removing your belongings from the property, turning off the gas, electricity, or water, intimidating or threatening you into leaving, and physically blocking you from entering the property. All of these are also forms of landlord harassment.
If you're illegally evicted, you should contact your local council's tenancy relations officer immediately. The council has the power to help you get back into the property and can prosecute the landlord. You can also call the police if you feel threatened or your landlord is using violence.
You may also be entitled to compensation through the county court for the distress and expense caused by the illegal eviction, and you can apply for a rent repayment order through the First-tier Tribunal.