Its a sauce that contains lettuce and vinegar to eat with roast dinner and can be eaten with chips and cold meat in Newcastle, UK.
I believe you are referring to coleslaw - it's actually cabbage rather than lettuce, and is considered a side-dish. It's often served with chips (French Fries here in the US) as an accompaniment to hot and cold roast meats as well as batter-fried dishes such as chicken, clams or fish. Coleslaw dressing can be creamy or vinegary in the US, and it's largely a regional difference - in Southern Cuisine, coleslaw dressing is primarily vinegar, northern recipes call for mayo or sour cream. Reviewing some recipes online, in the UK it appears that coleslaw tends to have both creamy and vinegary components to the dressing.