There is no such thing as a "false exempt card". No exemption card carries any legal weight.
children under the age of 11 (The UK Health and Security Agency does not recommend face coverings for children under the age of 3 for health and safety reasons)
people who cannot put on, wear or remove a face covering because of a physical or mental illness or impairment, or disability
people for whom putting on, wearing or removing a face covering will cause severe distress
people speaking to or providing assistance to someone who relies on lip reading, clear sound or facial expressions to communicate
to avoid the risk of harm or injury to yourself or others
police officers and other emergency workers, given that this may interfere with their ability to serve the public.
You don’t even have to show the card legally, it’s just for your own piece of mind.