All appliances in the UK have a fuse where they connect to the building wiring, normally in the plug, but can be in a fixed fuse-holder like this device. Somewhere in the process it was recognized that having lots of different fuse values would be confusing and awkward for users, so these fuses are the same size and always one of three standard values: 13A, 5A, and 3A. As noted elsewhere, you can buy these particular fuses in UK supermarkets and convenience stores.
If 3A is too high for the appliance then what the designer has to do is to fit it with a flex rated at 3A so that is protected by the fuse at the plug-end and then add additional, lower current, protection at the device end.
The UK system is clever and has subtle details like the standard fuse values which were good at the time it was introduced. But, it is also rather over-engineered, and not optimized for modern homes that have a lot of low-current appliances.