The circuit breaker is sized to protect the cabling of the electrical installation from overload, not to protect the actual appliances that are plugged in or connected. If an appliance requires its own overload protection it must be built into the appliance or designed into the appliance. For example many stoves have an internal wiring arrangement that prevents all the hob elements and the oven running simultaneously.
Yes, you can load a circuit to 100% of its capacity. There's sufficient safety allowances in the reference tables in the regs that dictate a circuits maximum load capacity so no additional blanket derating is required.
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