It's a reasonable question and is no secret as runway closures at London airports are published well in advance with details accessible through various web sites.
For example Heathrow are presently closing runway 09R/27L for 5 x nights/week between 2300-0600hrs local for full rehabilitation work, so they can use the other runway 09L/27R during this period and therefore remain open. Gatwick is the same as they can use the "standby" runway 08L/26R which can be used when the main runway 08R/26L is closed so can also remain open. With single runway airports it's more of a challenge to stay open when runway maintenance takes place.
Stansted introduced a reduced runway distance operation in mid-2000's so they could remain open for much of the time during overnight runway maintenance and have a phasing procedure working at each end, however if the work is in the central portion of the runway then a full closure is needed and this carried out at weekends when there is much less freight movements. Luton's runway is much shorter than Stansted's therefore reduced distance operations here is not possible and a full closure is needed.
Runway closures for maintenance which require a full closure are normally planned after the end of the summer season eg early November onwards and overnight when winter season traffic schedules are much less therefore minimising disruption.