In Britain, it's not common to ask for extra milk or cream when ordering soup in a restaurant or cafe. The typical British soups, such as vegetable soups or stews, are not usually served with milk or cream as part of the traditional recipe. If a particular soup is meant to have a creamy texture, it would already be prepared that way.
In America and other countries, the situation may vary depending on the type of soup and the specific establishment. Some soups, like clam chowder or certain bisques, are naturally creamy and may already include milk or cream as part of the recipe. In such cases, asking for extra milk or cream might not be considered rude.
However, for most soups that do not traditionally contain milk or cream, it could be seen as unusual or unnecessary to request additional dairy products. It's always best to be considerate and polite when making special requests at restaurants or cafes. If you have specific dietary preferences or restrictions, you can politely inquire if the soup contains any dairy ingredients or if there are alternative options available.
Ultimately, the key is to be respectful and understanding of cultural norms and culinary traditions when dining in different countries or regions. If you're uncertain about a specific soup or how it's traditionally served, you can always ask the server for their recommendations or more information.