Other mistake: Friday 12/8/11 Greg leaves for his holiday in Mexico by taxi. After he has gone you can see his pick-up truck parked in front of Dot's car in the square. On Monday 15/8/11, Greg's truck has gone.
Factual error: In the episode where Kat is flying off to Ibiza, she is seated in an over-wing exit seat. These seats are reserved for ABP's (Able Bodied Passengers) which mean no children or infants under the age of 14 can sit there. Once the plane has started boarding, check in closes for that flight and no further tickets can be sold for that flight. The captain requested pushback, but, certain conditions around the aircraft wouldn't make this possible. Stairs still in place, ground power was attached and once the door had been closed Alfie would have not under any circumstances been allowed onto a airfield to climb the stairs. If he did, he would have been escorted off.
Answer: Most likely a mistake by the set designers or it's a recycled set which already had the switch on (they just repaint and add other fittings).
At my workplace there is a two-gang switch with a three-gang switch underneath it, one of which is for the shop lights, the other four have no apparent use.