Approximately 2.998 x 108 ms-1 (to three decimal places) in a vacuum. But less than that in high purity glass (such as used in optical fibres) and significantly less than that in plastic optical fibres.
But... even if BT published that data in their advertisements, would it make the adverts any less silly?
My opinion is ‘No’. The fact that signals travel at speeds around 300E6 m/s, or more, or less, in air or glass or copper, is not a useful factor in describing a broadband service or the data rates it might support.
Edit: added ‘less’, to ‘less silly’.