Re: apple keyboard
The # symbol is inconsistently present on Apple's UK keyboard layouts across all products. There wasn't any particular date at which it arrived, or left, as the company regularly had some products with it, and some without, with no logic to which is which. The only vague pattern is that "Pro" in the name tends to correlates with "#" on the key-cap, but it's not a guarantee.
My own recent experience: Pro 15" (2015) - yes, USB Keyboard (2010) - no. Pro (2010) - yes. Pro 13.3 (2012) - no.
Consistency. It's what they're known for.
(Placement of # , ~, " and ` is the one thing that still catches me when I move from Mac to Linux/Windows. Objectively, the "PC" key placements are more programmer-friendly, but I have decades of muscle-memory telling me to type Option-3...)