So I guess US usage would be "Look at all my Legos". How does that work in singular? "Give me that red Lego"?

In the UK I've only heard Lego referred to like: "Give me that red brick", "Give me that red Lego brick", "Look at all my Lego", "Look at all my Lego bricks"

In the US usage, "legos" refers to many blocks. "Lego" is generally used as an adjective, referring to things constructed from many legos. "Lego house" or "Lego car", for example. It has been a while since I played with them, but to refer to a single brick, I would usually mention the size. "The red 2x4", for example, to refer to a red lego brick that has 8 pips.

Interesting point about the adjective, it's the same here. So the only difference seems to be when referring to a pile of bricks.

In the UK we call them lego because we consider it to be a mass noun, like sand rather than sands, and a grain of sand and a piece of lego.

I've heard "Legos" said plenty of times in England. So while it might not be correct it's hardly uncommon.

Weird. Lived here all my life, only time I've ever heard it is in "look at those wacky Yanks" conversations. South coast/London here. Maybe it's regional?

Trying to pattern match on my experiences, I might conclude that it's an age thing. People over 40 are more likely to say "Legos" than those under 30.

From a UK perspective. I'm 48, most of my friends are 38-60'ish, we all call it Lego. I'm more inclined to believe that it's younger people in the UK (under 25-30) that use "Legos", simply due to the availability of US kids telly that us older farts never had (I barely had three channels up until 2002 - BBC1/2 and Channel4). My ex's kids picked up some of these aberrations ("so fun" instead of "so much fun" is an example that used to grate for me). There were so many times I wanted to sabotage that bloody Sky box.

Like math? ;)