Brompton do ready made cable sets in the correct lenghts and afaik the cable inners are all Teflon lined ensuring this aspect is sorted, giving you a slicker cable that returns better than a standard inner. Essential on the long cable runs on a small compact folder.
If you want a slicker cable set up that will return better Teflon lined outer cables can be used, but these will need cutting to lenght. If you do this you need a good set of cable cutters.

Lastly add a touch of light lubricate even to a Teflon cable.