Could be a water resistant IP-- socket with a lift-up lid. Or a doorbell. Or just an empty box to confuse, befuddle (and electrocute) the new homeowner while trimming his bush.
It looks like an 'MK Masterseal' enclosure. The ones with the lift up lid are either sockets or fused spurs - some include timers and RCDs. It doesn't look like a switch as they don't have the lift up lid which I think I can just about make out the shut line of.
