"Axle boot pan" even considering different terminology between US and UK is meaningless. The output shafts on the rear axle have joints that are sealed with rubber boots to keep the lubricating grease inside the joints and to exclude dirt and water ingress into the joint. If the rubber was to either split due to ageing or external damage then replacement would certainly be advised. The grease that would leak out of the joint cannot possibly cause corossion to any adjacent parts. If you can get part numbers of the parts the Dealer is advising replacement then it would certainly clarify what is happening.