That was an illuminated switch. Containing a B22 holder for a pygmy bulb... I took one for scrap not long ago while trying to have a clear out.
Very crudely:
But this appears to be very low down, I assume that's the skirting in the corner, so I wonder if its a 'universal' enclosure for options in the range, possibly a switched socket except it seems to be extended to a couple more enclosures.
My guess, this was possibly the old heating system.
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