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? ;)