I would suggest trying Yellow Pages under anodisers and electro platers. Ideally you want to be able to go to the place and have a chat with them. Check what the place looks like and what finishes they can achieve.
The place i used was over in Feltham or Hanworth or Hounslow. Have had a look on yellow pages website but can't see the one i used.
The main thing is to make sure that they polish it up to a real shine... just like painting a wall. The prep work is what really makes the difference.
As i say, pick a few and go and have a chat with them