Some information here about mayonnaise - while it's a good source of fat, just beware that the carb counts can vary a lot. (And light/low fat versions are always higher!). I've listed some of the main brands (and some OL) below.
Many of them contain sugar, which is odd as if you make mayo yourself, you wouldn't add sugar. But then this is also true of many foods that are pre-packaged/processed. (As an aside, I was horrified to discover once that the brand of Chinese 5 spice powder I had bought also had sugar in it!)
The only brand that's zero carbs is Delouis, and this is because it's the closest to homemade - here are the ingredients:
Sunflower Oil, Dijon Mustard 9% (Mustard Seeds, Alcohol Vinegar, Water, Salt), Egg Yolk from Hens Raised Outdoors 8,85%, White Wine Vinegar, Salt, Lemon, Egg White from Hens Raised Outdoors, Contains Sulfites
It is, perhaps not surprisingly, also expensive.
Anyway, before you go dolloping great tablespoons of the stuff on to your plate, just have a look at these
Carb counts, per 100g:
Delouis - 0g (available in Waitrose)
Heinz Seriously Good - 3g (Light version is 8g)
Hellmann's Real - 1.4g (Light version is 6g, Lighter than Light 9.7g)
Hellmann's Vegan - 3.9g
Hellmann's Organic - 1.6g
Nando's Peri-naise (mild) - 13g
Sauce Shop Ranch Mayo - 2.4g
Stokes Real Mayonnaise - 2.7g
Leon (Korean) - 21.4g
Sainsbury's Real - 3.5g (Light version 9.9g)
Sainsbury's SO organic - 6.7g
Sainsbury's Stamford St - 12.1g
Tesco Mayonnaise - 1.9g
Stockwell (Tesco) - 11.1g
Tesco Lighter than Light - 12.5g
Waitrose Essential - 3.3g
Essential Half Fat - 10.4g
Waitrose with Olive Oil - 3.4g
Aldi Bramwells Real - 3.4g
Aldi Branwells Light - 6.1g