I think H&S is being used as a red herring here, these service guy's are supposed to be trained personel, but incapable of disconnecting a fused outlet unit, beggars belief!!!
The easiest way to get these service contractors to honour the manufacturers warranty is to go to the CAB who will contact the said company and inform them that they are in contradiction of consumer rights law. It's NOT the manufactures that throw this crap up, it's their contracted out service companies.
I've never seen any appliance manufacturer's warranty data, stating that removal of the supplied plug top, will void its warranty, basically because it would be in breach of UK's consumer law...
Any appliance that has been connected to the main supply via a connection method recognised and in accordance with BS7671, it's warranty cannot be voided, it's as simple as that.