I don't think UK regulations stipulate a particular fill factor.
Good practice, the ability to draw in with relative ease and perhaps leaving room for later additions, would suggest leaving a reaonable amount of space, but the exact figure will depend on the circumstances (e.g. duct length between acess points and number/angle of bends etc.)
Tables 4C3 and 4C6 will give a general idea of rating factor that is more likely to be the determining criteria rather than how many you can stuff into the duct/trench.
