While LHR has 27L & R as "preferential" runways, to share the noise around there is a published list of dates and runways alternations to be used.
It isn't uncommon (especially at the moment with all the work being done during the night) to be using one runway (say 27R single), then if the work is finished in the wee hours of the morning, that the other end is officially the one that should be used (taking into account wind direction and strength).