ICFC stands for Industrial Commercial and Financial Corporation, which was a post-WWII venture capital group launched by the Bank of England. There's a decent overview of this if you do a simple wikipedia search. By the mid-1980s the group had changed names and was no longer operating under the ICFC flag.
So, back to the whisky, your nip was bottled before then, and would have been a private labeling for ICFC. It was probably done as a corporate holiday or entertainment giveaway. But it brings a smile to my face to imagine the bottling may have been done for the bankers simply to have in their desk whence in need of a pick-me-up during a tough market day!
As to its worth, there may be a banker out there willing to pay a ransom for this bit of history? Who knows, perhaps Macallan may even be interested in acquiring it back if they felt it was historically relevant to their library?
Of course, I'd drink it next to a new Macallan bottling to try and see how their spirit has changed over the years. Which is precisely why I don't have a whisky collection and instead one of empty bottles!