I can't speak for the UK or Japan HQ's, but Pokemon's HQ in the US just outside of Seattle is in a mega office building, with no indication whatsoever that the Pokemon Company was there. Granted, they share a building with companies like Microsoft and Valve, but they aren't even listed as one of the major tenants of the building. The only reason I know all of this is because I thought it would be fun to see where it was last time I was in Seattle, and did a bit of research before realizing that it wouldn't be worth my time going out of my way to look at it.
And again, only speaking about the US HQ, but your second description is a little closer to what the inside of the building looks like (found some pictures of it
here). Can't say I'm all that surprised, but disappointing that it isn't a little more colorful and... Pokemon-ish.
BTW, I'm going to move this over to our Pokemon General section, as it's a discussion about the franchise as a whole, not just the games.