MD
Hinting would be my first guess. Firstly you would design the character and style of the typeface for print use. Then you would add instructions called Hints, which tell the font renderers built into OSs, how to fit those characters into pixels on the screen. Then there is TV/On-Screen use. Whilst these could also count as pixels, the live character generators like VizRT etc, could have some quirks to how it puts stuff on the screen. Typically the rendering on these systems is optimised for speed and not accuracy, so tweaks to the glyphs may be needed to ensure they look good at various sizes, and when moving etc.
Ligatures may also be added over time, as well as new currency symbols. Also with BBC Global News in particular, they will need to ensure the Reith fonts gel well with the foreign language typefaces they use, in terms of character heights, x-heights, so when the languages are used together on screen, it does not look odd. I could imagine the BBC commissioning additional character sets for various world languages so you get BBC Reith Urdu, and BBC Reith Arabic - to further reduce licensing costs for other typefaces they currently use. These additions will take time and money to get right.
Ligatures may also be added over time, as well as new currency symbols. Also with BBC Global News in particular, they will need to ensure the Reith fonts gel well with the foreign language typefaces they use, in terms of character heights, x-heights, so when the languages are used together on screen, it does not look odd. I could imagine the BBC commissioning additional character sets for various world languages so you get BBC Reith Urdu, and BBC Reith Arabic - to further reduce licensing costs for other typefaces they currently use. These additions will take time and money to get right.