3) Vacuum-Metalising - The most widely used technique and is common on a number of household goods and toys. If you've got anything at home thats made of plastic with a chrome finish then chances are its been VM'd.
So given a number of constraints I went down route 3, Vacuum Metalising and have found a factory in the UK which specialise in this field doing a considerable amount of work for businesses in the area including the Local Authority where they produce the reflective "mirrors" used in traffic lights/signals, they've also produced some helmets for the upcoming Brad Pitt/Orlando Bloom movie "Troy".
The process for VM is 4 stages. Firstly a base lacquer is applied to the prob. Secondly its baked in the oven at 60-70 degrees. Stage 4 is the chroming part where the prop is placed in a Vacuum chamber and is bombarded with vaporised Aluminum, and finally a top lacquer is applied, and in the Case of C-3PO it has a Gold tint.