At Heathrow with the parallel runway setup of 09L/27R and 09R/27L, the operation runs with using one runway for arrivals while departing from the other. On the 27 operation side of things, the runways are "alternated" to share the noise (supposedly) while on 09 operations, due to the Cranford treaty (and the ground environment) 09L is the landing runway with departures off 09R.
Mixed mode will basically mean the use of both runways for both arrivals and departures. However, as the runways aren't sufficiently spaced, parallel operations of aircraft flying down the ILS side by side isn't always an option, especially considering the normally lovely English weather
.