Right, like I said when getting on a plane to the UK all you need is your passport. When you arrive and they question you that is them approving or denying your visa on the spot. The landing card is part of the process. They don't require a mass amount of information from you if your story for coming to the UK seems plausible and they don't want to put tourists, which generate a lot of money, through a lengthy process. If you do anything wrong while you are there, it terminates your visa.
When you plan on staying longer, the visa process gets longer and requires more information.