Agree, but what's important is the orders, would the UK keys stop you from ordering the base? I doubt it. But I wouldn't consider a set without them.
On the cost side it's up to the designer to negotiate a right price for themselves so not my place to comment
I mean, the same case could be made for any extra keys that someone could ask for. For example, I rarely buy sets that don't include certain keys I need for the layouts I most commonly use, so I understand where you're coming from. But that doesn't mean that including those keys is always a good idea. This kind of argument is flawed: Sure, UK keys wouldn't prevent anyone from buying the set in and of themselves, but those keys don't come for free, and so the additional cost might.
The question is whether adding those 4–6 keys would be worth the price increase to everyone's base kits. This is the bottom line, and it's reflected in the number of orders, as you said. The answer to this is likely no, as ISO-UK users only make up about 2–3% of all users (
source).
There's not much a designer can do to reduce the price of a larger kit, other than go with a higher MOQ (which has its own risks) or forgo their profit margins. It's in (almost) everyone's best interest that the kits be well optimized.