I think it's a North American thing to call anything LEGO "Legos", denoting that it is a thing made up of many pieces of "Lego" right? In Europe if you pointed to a LEGO set or a pile of LEGO pieces every response you'd get would be "that's LEGO". But then again in Europe, or at least in Germany, we had many different brands to choose from for the same kind of construction play, whereas in North America it seemed like it's just LEGO.
I agree, in the UK I never heard them referred to as "Legos" and only ever heard that in American media. It always felt like one of those sloppy contractions Americans use, like "Do you wanna go with?" and "I'll write him"
One in France (or other European French-speaking countries) would say "des Legos" to refer to a bunch of Lego pieces or Lego sets. "Un Lego" would mean a single Lego brick.
There are several different brands of LEGO-compatible bricks in the US. In practice, though, the bricks are called "legos" generically, whether they're LEGO or not.