I disagree vehemently shunt old chap/chappess. Salad cream is a gift from the gods and contains the right amount of vinegar to make an unrivalled piquant dressing. I don't think it plebian at all to have s/c on your salad instead of a new fangled balsamic and pinenut dressing. Mayonnaise on the other hand is thick, gloopy, tasteless and fatty (home made is slightly better) and has to be WITH something to make it work (hence the preponderance of shop-bought butties having gallons of the stuff glooped on each slice (BLT AND mayo? - come on - are we Yanks or what?)). Salad cream wins hands down on account of the vinegar content to lift and cut through the egg and oil combo.
It was the first British Heinz product to be developed exclusively in the UK, back in 1914 and was created specially for British tastes at the time It took eight years to perfect and is, quite literally, the business.