"with the required UK-US conversion keycap set arrived"
The problem is when you can't do that - in you case it was rarely used keys, but maybe for accountants. In other cases, you can't remove needed letters. The issue is many languages have some more glyph than English (i.e. accented characters, language-specific special characters like, etc.), and they need to find a place on a keyboard - and old keyboards had often less keys.
As long as a keyboard is only used on machine for very specific use, you can switch to a US one, but then it becomes difficult when you had to write in your native language - I've seen many programmers using US keyboards, but then producing very bad documents when they couldn't use the proper glyph and used instead something resembling it, as done on very old typewriters (i.e. using e' instead of è)
IBM started to delivery localized keyboards through its typewriters business far before it started to sell computers. I guess they had a far broader view of what characters are easier to use across different layouts, than some programmers who never put their noses outside US (but to get intoxicated in Mexico, maybe).