Aren't UK sales for GoW around a couple hundred thousand? Because they're closer to 2 million in the US. Doesn't sound like a mini-US at all.
And yes it does mean something. As Mezoly said, it sold less than HZD and UC4 in the UK, that much is a fact. Yet it's on the tails of HZD's year plus sales in less than a month. That alone should be indicative that it's incredibly misguided to put much stock into that single region alone. And you're talking legs as if you have reason to believe Detroit won't have legs when there is no indication of such. You also say GoW is a monster in the US but why are you arbitrarily ignoring the fact that the same could be the case for Detroit? Heavy Rain did amazing there as well.
And with threads like this:
https://www.resetera.com/threads/da...far-quantic-dreams-biggest-launch-ever.45637/
Cement the fact that you're not looking at the situation properly at all if you assume UK means so much for the performance of this game.