It's LEGO-fan usage more than British/American difference. A fan always says "I play with LEGO, this is a LEGO brick, hand me some of that LEGO" whereas a casual user might say "Scoop up some of those LEGOs; I play with LEGOs."
Also, depending on the particular American accent involved, I often find the pronunciation quite unusual. Sounds more like "lay-goes" than "leh-goes", similar to the way some Americans pronounce the word "leg".