Although it is extremely solidly built, it's mostly built for a different context: the UK mostly uses ring circuits (similar to a ring network) rather than the radial topology used pretty much everywhere else. This means there can't really be "small" fuses at the fusebox, you can't put a 16A fuse for the entire ring because that'd be the load for all the devices in the house.

As a result you need smaller fuses at the plug level. An advantage of that is that your fuses can then be sized specifically for the appliance cable, and thus you can safely use much smaller cable e.g. let's say that you want to design a 2.5A extension cable, you can put a 2.5A fuse in it and be safe. In a radial context where plugs are unfused, somebody could plug a 10A appliance on the 2.5A cable and melt the cable, the fuse only protects the "wall" circuit and won't blow until >16A (generally).

In fact this has been an issue in the US in the last few years: because of the low voltage, 20A circuits have become more common, but extension cords are still usually 16A. If you plug a 16A cord to a 20A circuit and draw 20A from it, the fuse will be perfectly happy (everything's fine as far as it's concerned) but the cord can catch on fire.

Anyway since they needed larger plugs for the fuse, I expect the UK standards body reinforced the plug while at it.