Are they legal and do they comply with British Standards?
In short, yes, chrome fire extinguishers are completely legal. The confusion over their legality arises from whether or not they conform to British Standards, specifically BS EN3-7: 2004. Part 16.1 of the Standard states that, in order for a fire extinguisher to be BS EN3 approved, 95% of the body of the fire extinguisher must be red.
Therefore, chrome fire extinguishers fall outside of the scope of BS EN 3 and are not compliant. However, the BS EN 3 standard is a recommendation, rather than a law, and chrome extinguishers carry other approvals, so you are not breaking any fire safety regulations by installing them.
Instead, the governing fire extinguisher legislation is the CE Mark. From May 2002, all fire extinguishers which operate under pressure must be manufactured in accordance with the European Pressure Equipment Directive (PED) and those which are will feature a CE Mark. Chrome fire extinguishers all carry a CE Mark, ensuring full legal compliance.