NICEIC (National Inspection Council for Electrical Installation Contracting) is the UK's largest voluntary register of electrical businesses. They offer a find a trusted tradesperson scheme. They also run the Microgeneration Certification Scheme, where people can find a Find an MCS Certified Installer.
Go Renewable is a site by the Energy Saving Trust and MCS. People book a home energy survey, which is then used to get quotes from MCS installers. Unfortuantely, currently restricted to the south of the UK. People outside these areas can still use their online suitability tool
The Smart Export Guarantee requires some electricity suppliers to pay small-scale generators such as households for low-carbon electricity which they export back to the National Grid. Your installation needs to be certified, for example by MCS.