If you wish to terminate the tenancy then you can do so at any time, and for any reason, provided you give the minimum notice.
The notice period is three months for an unfurnished letting, and one month for a furnished letting.
Clearly, although may do so in any form you would be best advised to ensure it was in writing and, in the case of doubt, sent by recorded delivery.
The landlord is unable to terminate the tenancy before the end of the term.
A tenant of an unfurnished letting has the right to a minimum term of 3 years, while it is one year for an furnished tenancy.
The notice period the landlord must give is six months for an unfurnished letting, and three months for a furnished letting.
There are strict procedures with which the landlord must comply in order for the notice to be legally valid eg evidence of delivery, date of notice, specific reason, such as sale of the property.
If the reason is sale of the property, then the tenant has the right of first refusal. Again very specific and strict procedures apply.