Nice bag, and there is still a lot of life left in it! I agree that I would start with a long soak and vigorous bath in Ivory or Woolite. I have never used Dawn but it is supposed to work well on grease spots - but I don't know about that. I would shape the bag and stuff it with towels while it is drying. After it is dry I would condition the leather and polish any hardware.
I buy Apple Conditioner at the Burlington Coat Factory and Leather CPR at the Tuesday Morning Store. I buy Wenol metal polish online and I get most of my other leather care products at a local tack shop (equestrian horse-riding store.)
I usually find British Tan bags like your bag quite challenging to clean and condition because it is a lighter-colored leather and it shows spots, stains, and darkening in "high-touch areas" like straps and around turnlocks and closures. Such discolorations often persist after cleaning.
I usually resort to Black Rock Leather n Rich to condition British Tan bags. It can be a little difficult to work with but it does darken leathers and that helps to even out inconsistent coloring.
The link below includes a summary of the leather care products I frequently use and my opinions and tips for their use. Good luck, that's a lovely bag so give rehabbing a try!
http://forum.purseblog.com/coach/ca...oner-opinions-please-697350.html#post19580337
I didn't have much experience with Black Rock when I wrote the post at the above link but I have since learned to use it more efficiently (thanks to Docride at the Hermes forum) and now I like it a lot for bags with inconsistent coloring. Here is some more information about the best way to use Black Rock:
http://forum.purseblog.com/coach/coach-rehab-and-rescue-club-624452-37.html#post19792894