I miss the days before AMS opened everywhere. Back when you had to sign up for a special account, AMS was so much less competitive in non US regions. The UK bids were even lower back then!
The UK market is harder to penetrate, for whatever reason. It is genre dependent of course, but if you look at the top 100 in the US vs UK, or you check the relative ranks in the genre top 100, you'll see way fewer indies. Even highly successful indies.
Look at the top 20 in contemporary romance right now. Those books aren't in the top 20 in the UK and are often ranked much, much lower (they might be #17 in the US and #450 in the UK).
The UK market has a much lower ceiling than the US and it doesn't seem as KU centric.
I've had some books do especially well in the UK and they often had more upscale packaging, but I don't think the stories were necessarily better written or more upscale. Some were pretty dirty.