That looks like an early
coach bag designed by Bonnie Cashin, where did you get it? It's really gorgeous!
You should post it in the Authentication forum to get it authenticated; does it have a tag inside that says "Coach"? Here is the link for the authentication forum and information about what should be included:
https://forum.purseblog.com/threads/authenticate-this-coach-see-first-post-for-format.889527/
A big ink spot like that is just about impossible to remove. It appears to be on the inside, correct? I'd probably leave it alone.
I would want to give that bag a bath but I'd be worried that the ink would run but perhaps another rehabber has had experience rehabbing a badly ink-stained British Tan bag and will be able to advise you.
The rehab process is pretty straight-forward, aside from that ink spot. Use Dawn dish soap and soak the bag in sudsy water for about 20 to 30 minutes, vigorously massage any dirty areas to clean. Rinse is plain water, drain, stuff with clean towels or rags to shape the bag so that it will dry with the correct shape. When it is half-dry remove the towels and apply Leather CPR to the leather surfaces. After it is completely dry, rub in thin applications of Black Rocks Leather n Rich, let it sit for at least a few hours and then buff with a soft cloth or horsehair brush - that will hide most of the scratches and blemishes.
You can use a metal polish like Wenol to polish the brass fittings. I cannot tell from your picture but is there any dark green gunk around the lock and/or metal fittings? If yes, then the metal is afflicted with verdigris and will need special treatment, but we can discuss that later.
I've only rehabbed one vintage Cashin bag but you can see before and after pics at the link below. No ink spots but otherwise it has the same coloring as your bag.
https://forum.purseblog.com/threads/proof-that-a-50-yr-old-lady-can-still-be-beautiful-rehab-reveal.742282/
NIce find, good luck!