In 1899 the London Government Act divided London into twenty-eight Metropolitan Boroughs, which lasted until 1965 with the creation of Greater London and the thirty-two London Boroughs which we know today. Signs from the Metropolitan Boroughs still exist in many parts of London though, such as this Borough of Chelsea sign. Postal districts were created in 1857, but weren’t numbered until 1917. So it seems likely that this sign dates from between 1899 and 1917. The lettering of the street name and postal district is of a style that seems quite specific to signage, especially enamel signs (though this sign is milk-glass not enamel). It’s a condensed sans that features angled terminals on certain letters – the C, G, J and S – but I haven’t yet found out where it originates from.