As @londongirl12said, for jam/butter type things put on the scale, take our your portion and reweigh. The negative number is how much you have on your plate. Works for salad dressing, honey, milk, loads of things!
I use nutracheck which contains calories for a lot of UK restaurants but if the specific restaurant isn't there then I would search the food database for a similar restaurant dish. It's close enough (unless you are eating out a lot).