I was doing some wiring on the car over the last couple of days, and had to have the passenger Kick panel off.
So I checked out the Relays for you....
I have the one fixed to the panel like yours and an Empty plug like yours, so I would make a guess that the socket in not used on the UK Spec.
I have most of the gadgets as standard on my car, but do not have heated seats, headlamp washers, or Dim/Dip Headlights, so I would guess it is for one of those extras.