Good questions safe + sound,
I'd agree that desk top PCs could be classed as movable equipment in many situations but where I work there is great aversion to shutting them down so it is quite common for a user to move a PC from desk to desk, or dump it on the floor to clear some room, without unplugging it. I think this is bad practice but as neither the IT staff or I can control it I use the preset portable equipment test with the 0.75mA limit. With a liberal interpretation of this limit I've only ever ended up failing very old or genuinely faulty PCs so no ones got upset.
The earthing facility of a BS1363 plug is only designed to be safe for a maximum of 3.5mA earth leakage which is why the CoP gives this as a maximum and asks that equipment exceeding this comply with the 'High protective conductor current' provisions in Section 15.11 (or would do if they bothered to proof read it properly
). The risk of the inadequate earth, in particular the single screw terminal in the plug, failing and exposing the user to touch current in excess of 3.5mA is just as real whether the plug be connected to a single high leakage appliance or several lower leakage appliances whose design leakage currents add up to more than 3.5mA.