I have never heard of any rule that you cannot go into a restaurant and order dessert only. There might be a problem with buffets. (The only real exception would be V&A which only has a fixed-price meal.)
Whether or not you need a PS depends on the time of year, day of week, and the restaurant. If you don't have a PS you may not get in, or will have a long wait. If you do get one, you will allow them to turn the table faster.