Updating post from Reddit.
For freestanding electrical appliance repairs such as fridges etc. at a rental property, is there any certification or qualification that the repairer should have?
No. There are no mandatory certification/accreditation requirements for repairs to most free standing electrical appliances at residential homes.
The only possible exception are fridge or freezers that contain F-gas, in which case the contractor must be certified to work on appliances containing F-gas
It is however still a good idea to check that the contractor belongs to a UK accreditation body for electricians e.g NICEIC or NAPIT.
Additionally, you can check they also belong to a Health and Safety accreditation scheme such as Safe contractor or CHAS. These are expensive memberships though & smaller contractors may not have this.