Breaking an agreement early
If a tenant wants to end their agreement early they should give as much notice as possible, preferably in writing (keep a copy of the letter). It is a good idea to state the exact date you intend to leave and that you want the landlord (or agent) to find a new tenant. Any assistance in finding a replacement tenant (such as making the property readily available for inspection) may help to reduce the costs involved.

Having said this, breaking an agreement can be costly
A landlord can claim compensation for any loss they suffer as a result of a tenant ending the agreement early. The costs a tenant could be liable for include:

• rent until new tenants move in or the existing agreement runs out (whichever happens first);
• a reletting fee (usually one weeks rent) when the property is let by an agent who charges the landlord a fee for finding new tenants; and
• advertising costs

For a landlord to successfully claim, they must be able to show that their loss was caused by the tenant breaking the agreement early, not by other factors.
For example, if a tenant breaks the agreement just prior to the expiry date, the full amount of reletting and advertising charges may not be able to be passed on since the landlord would have incurred these expenses shortly anyway.
The landlord also has a duty to keep their loss to a minimum. This means that the landlord must make a reasonable effort to find a new tenant, otherwise any claim they later make may be reduced by the Consumer, Trader and Tenancy Tribunal.