Require that all new fridges fitted in food retail locations have doors fitted (especially supermarkets with highest number of fridges) and give timeline for replacement or upgrade of all open-front fridges.
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The food retail sector is responsible for approximately 3% of all electricity consumption and 1% of greenhouse gas emissions in the UK, and in-store refrigeration is responsible for around 50% of that consumption: https://www.imperial.ac.uk/news/197495/research-suggests-fridges-could-reducing-supermarket/
In-store refrigeration can be made 30%-50% more efficient by fitting fridge doors: https://eia-international.org/wp-content/uploads/Chilling-Facts-VI1.pdf
This means fridge doors could save us around 0.6% of our total UK electricity requirement, and reduce UK GHG emissions.