Absolutely not! I never said that a higher rated fuse MUST be used nor did I even infer it. I suspect you knew this already.
Boilers (or indeed any electrical devices) requiring a 3/5 Amp inline fuse, to be within regulation, as you also know, are commonplace, however these fuses are NOT there to protect any components within, other than the light gauge wiring used within such appliances from overheating/catching fire, should a high current be called for through a component failing.
A properly designed PCB handles overloads in a totally different way than relying on a basic fuse, to protect itself.
A fuse is a very crude safety device and even a fuse (generally quick blow rated) mounted on a PCB is there to prevent ALREADY failed electronic components from drawing currents that may lead to overheating and hence a potential fire risk.
To sum up, if any device is blowing a fuse (main board/plugtop/spur unit) that is rated to be capable of carrying that devices expected current, then there is a fault within said device ie. the fuse did not "protect it". The blown fuse however has stopped unknown currents being drawn and thus heating up supply cables to the point of causing a fire.
However we are drifting away from the OP.