It’s good safety practice to reinspect portable appliances included in the tenancy agreement before the tenancy changes and the new one begins. After, all small appliances will need to be tested every two years. Larger appliances, refrigerators, freezers, washing machines etc., should be tested every four years.
According to the Electricity at Work Regulations of 1989, all electrical equipment that could potentially injure people must be kept safe. The Regulations, however, do not specify what must be done, by whom, or how often (i.e., they do not make electrical appliance testing or inspection a legal need, nor do they make it a legal requirement to do this once a year).