A factory fitted option is an addition (modification) to the basic specification , so I would certainly declare it. See above re the ally wheels on an MX 5.
If in doubt tell your insurer, it's highly unlikely to effect the premium, but quiete possible it might affect any claim. Like I said, if in doubt, declared it, why take the risk?
Any deluxe model has extra items as standard, and such items, (sun roof, leather upholstery etc) is included in its original specification. No passenger car that I can think of comes with a towbar as standard (except perhaps a Disco or similar?) So as such any towbar is a modification from the original specification, and thats the attitude most imsurers adopt which is why it should be declared.
To quote that old Athna poster of the young lady playing tennis "If in doubt, check!"