In case you wonder why, UK domestic circuits are wired as a ring main (a naval practice) which means you can deliver more power with less wire and are typically fused at 60 amps. The plugs are designed to deliver 13 amps with ease and that is the maximum fuse allowed. They are supposed to be fused to match the load they are being used for. A plug adapter for a PC, for example, would probably not need to be fused at more than 3 amps to handle the initial inrush.