According to the following website, it should be on offside or centre & it does not mention incorrect position as an MOT failure. It seems like they only test the offside one. Many cars these days only have one foglight, although it used to be common to fit two. Often, you will see there is a space in the lens where a bulb could be fitted, but as manufacturers started to cut costs, the bulb holder was often deleted.
Good link, Simon.
I put a flush fit foglight in the centre, to keep it away from blinding drivers to the brake light. The advantage of that
might be outweighed by the fact that a tow-ball could obscure it if I fit one, and reversing onto a trailer would probably smash it!
With hindsight, I would have fitted a flush foglight on the offside, and got a clear-lense version for the nearside, wired to the reversing light.
Got my light from ebay, but have heard that AVA and the clubshop do them, a Mazda car switch fits the dash holes, and looks neater.
Hope this helps,
SD