The British ton is 2240 lbs. and, as PRR mentioned, it can depend on when the rifle was made AND what it was proofed for and with at that time. It would be good to see all the proof marks to hopefully establish a date.. The bluing...it does look buffed to me. Value, I don't know, Rigby's are not cheap, even if refinished. I would have to call that a "re-finish" and not a restoration. I'm not certain the stock is original either.
I think PRR gave the best advice, contact Rigby. If the records still exist, and I believe they should, they can tell you a great deal about who, what, when and where.
Edit: I did some browsing of various sites as to what Rigby bolt rifles are listed at and selling for. There was several in 375 H&H. I did not look at all the pictures or enlarge all those I looked at. What I saw was one for less than 10K and, it was $9,900 and some change. Some were in the 50K range...but I don't think this rifles falls into that price level. I suppose it could if a special provenance could be established but, we don't know of any right now.
I have no idea where 'newb's rifle would fall. A Rigby letter would shed some light and add to the value as would seeing the rest of the proof marks. Regardless, it IS a Rigby. They won't be found in the vast majority of collections or accumulations.