This appears to be the same whisky bottle, but filled to the proper level. Makes me think your bottle has been opened.
Google is your friend. These guys
are ******* a bottle for 3,900 pounds.
There are 70cl bottles that are "1938 35yr Rare & Fine" and "1938 31yr Rare & Fine" for closer to 10,000 pounds. These don't appear to be the same as what you have, as your bottle is 75cl.
also seem to have your same bottle for ****.
They have it listed for 3,500 pounds. The image they have shows a bottle filled to the proper level. If your bottle IS sealed, then the cork must have been damaged, allowing some of the liquid to escape or evaporate.
SO, if you had a pristine bottle, and sent it up for auction, I would assume you could fetch 80-90% of the retail store price. This would include auction fees. If you **** it privately, you could assume to fetch 60-70% of the retail store price. Because your bottle is not full, it is indicative that the seal has been broken, and the cork is damaged. I would fear the alcohol has all but evaporated out of the bottle. I would pay exactly nothing for the bottle. If, the seal is still good, but your friend has opened the bottle and tasted a little, then the authenticity of the bottle is compromised. It could have been refilled with Cutty Sark for all I know. I would pay exactly nothing for the bottle.
However, if you can find someone who has TASTED that same bottle (and presumably loves it), and would be willing to buy the opened bottle after tasting it to confirm its authenticity, you could sell it to that person for drinking purposes, and not as an investment. This would fetch a far cry from 3,500 pounds.
I'm sorry to be the bearer of bad news, but you seemed to be very persistent about getting SOME information.