The law provides certain protection for 'residential occupiers' from unlawful eviction or harassment in relation to premises. You are a 'residential occupier' if you are occupying premises as a residence (i.e. you live there) either under a contract (a tenancy agreement) or a particular rule of law or legislation giving you a right to remain in occupation.
It is a criminal offence for any person to unlawfully deprive you, as a residential occupier, of your occupation of the premises or part of the premises. Changing the locks would be an unlawful eviction unless you have been officially evicted from the property by a court order or the landlord can prove that they had reasonable cause to believe that you had ceased to reside in the premises.
An unlawfully evicted tenant may use reasonable force to regain entry to their own home, for example, breaking a window, though this will always be judged on an individual basis. If you had been lawfully evicted, you may be committing an offence of criminal damage.
The Tenancy Enforcement or Environment Protection Departments of your local authority, a solicitor or the Citizens Advice will best be able to advise you.
To find your local authority, please see the link in Related Information.