Wikepedia suggests it is similar to another product
"The H. J. Heinz Company remains the primary producer of salad cream, although it is also produced by other companies in the United Kingdom. In the United States, where salad cream is generally unavailable (and unknown), the T. Marzetti company produces a similar product, called T. Marzetti Original Recipe Slaw Dressing. Salad cream generally comes in a clear glass bottle, or, more recently, a clear plastic squeeze bottle."
Ohhhh okay! I know Marzetti's slaw dressing. It is very good, but I only use that on cole slaw, not a lettuce salad.
That makes sense -- coleslaw dressing is runnier than mayo, in the States.