British English speaker here - Lego was always a collective noun for me, like sheep. You might have a piece of lego, and some lego, but never 'a lego' or some legos.

It was oddly jarring to come across Americans on the internet, who had a totally different take on this!

(yes I realise you can have "a sheep", my example isn't quite right! Perhaps it's more like sand or rice)