I believe this is almost exclusively a Telegraph thing, and even then mostly in headlines. From their style guide:

“Percentages: per cent does not take a full point. Use pc only in headlines and % only in tables. In City page copy pc is acceptable.”

The same section of the style guide chooses to use hundredweights as an example unit of weight, so I definitely think there’s a significant degree to which they’ve just always done it and do not want to change it