All you need to do is get it plated nice and shiny gold like this.

Essentially there are three techniques:

1) "Proper" immersive Chroming / electroplating - After discussing this with a number of factories in the UK my understanding of this is only a very limited number of plastics will work with it as the overall process is so aggressive.

2) FantaChrome / Cosmic Chrome - 3 part spraying technique similar to mirrors. Looks great according to the sales literature but I've not had anyone tell me that they've had great results from it.

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. Sounds easy?

REMEMBER - ANY kind of Silicone contamination will destroy the finish of your prop  - leaving an "Orange Peel" type finish also reminiscent of oil on water. Essentially Silicone acts as a barrier (as its a perfect lubricant) stopping the lacquer adhere to your prop. You can not "remove" silicone if its on (or in) your prop and believe me I have tried every so called "Silicone Remover" possible! 

1) Been cleaned with soapy water, sanded, washed again and cleaned in IPA
2) Been VM'd (giving an appearance similar to above)
3) Been totally stripped down in a detergent and re-sanded.
4) Cleaned again in IPA, Mentholated Spirits and also a new German "Super De-Siliconiser"
5) VM'd for a second time

In addition I suggest EXACTLY the same approach as I took with Steve's helmet namely,

1) The front and back half's were filled and sanded such that when put together they fitted correctly without an obvious gaps.

2) As the eyes don't need to be VM'd (they can be painted with a copper paint) they were removed.

3) The neck bolts and head antenna were not affixed but supplied such that they cold be VM'd separately as a higher temperature (one that would melt the helmet if done together).

4) the helmet was lightly sanded with 600-1000 grit wet and dry paper

5) The helmet was then given a "generous" spraying of 2-pak primer to seal the silicone contaminants in. Following this it was very lightly sanded with 1000 to ensure a clean smooth surface. However Cyberman has suggested this be increased to 2000 Wet and Dry.

6) It was then cleaned with IPA, and bagged and then placed in a clean bag.

7) At the VM factory it is mounted on a rig using the holes in the neck to secure it to the rig which will later rotate the helmet inside the Vacuum chamber. For this reason you cannot bolt to the halves together.

8) The VM Process begins with the application of a base lacquer 

9) Its then baked on at 60C, the metal parts baked separately at a far higher temperature and following this they are mounted on the same rig as the helmets.

10) When cooled the rig is placed in the vacuum chamber where the Aluminum strips are heated and vaporised as the helmets rotate around inside the chamber. The gaseous Aluminum particles condensing on the helmets at a thickness of just 1 thousandth of an inch.

11) After some time the helmets and metal parts come out of the VM chamber looking silver. They are subsequently sprayed with a top lacquer which has a gold tint giving the impression the whole helmet is made of pure gold.