Your tenancy agreement will state that you need to give written notice but some landlords will accept a tenants notice to quit by email and even phone but it is very few and far between and is mainly private landlords.
Most contracts state that notice has to be in writing and posted to the landlords or agents address as stated on the tenancy agreement.
Certainly letting agencies will not accept anything other than written notice by the tenant. Section 19A of the Housing Act 1988 which forms a AST (assured shorthold tenancy) states that the tenant has to give the landlord 1 clear months notice dated from the start of the tenancy agreement. For example if the tenancy agreement started on the 12th of the month then the notice has to be from the 12th of the month. If notice is given at any other time after the 12th then it will not be deemed as one months clear notice.
The landlord has to given the tenant 2 clear months notice. This will be in the form of a legal document as outlined in the Housing Act 1988 and it is called a section 21. Like the tenants notice, the section 21 must start on the rent payment date and be 2 clear months. A section 21 cannot be emailed as it is a formal written document and needs to be served to the tenant or the premises that the tenant occupies.
In summary, if you are a tenant for your own peace of mind it is safer to give your notice in written form by post.