Can Of Tomato Soup in England
I would expect a British recipe to specify whether the soup is condensed (usually Campbell's brand) or ready to eat (after heating), examples are Heinz, Baxter's or any of the many supermarket own brands. So you need to specify what the recipe calls for. If it's condensed, the tins contain 295ml of the concentrate. The other sort comes in several sizes, but if a British recipe calls for "a can" I'd say it's likely to be 400ml, unless otherwise specified.