It’s not illegal to sell unsuitable fittings and appliances, but it is illegal to install one, so check suitability before you buy. The WRAS can tell you what’s been tested and complies with the Regulations, find out more in their Water Fitting and Material directory.
Double check valves (DCVs) on household appliances
“Domestic unvented primary circuits may be filled or replenished by means of a temporary connection between the primary circuit and the supply pipe. An ‘Approved’ double check valve shall be installed at the point of connection to the supply pipe. A permanent connection between the primary circuit and the supply pipe may be made via a CA backflow prevention device or some other no less effective backflow prevention device.”