I am Canadian myself and generally follow the American style, but I believe it is very common, though not universal. Scanning The Guardian, they (!) seem to follow the American style, though this paragraph popped out at me where they use both:

> Labour [singular] takes comfort partly from the fact it expended little effort or money on the seat, allowing the Lib Dems [plural] to declare themselves in the best position to challenge the Tories.

But an American publication would probably write the same, because the name Lib Dems is itself pluralized.

"Labour" is a party name, and it is singular. "Liberal Democrats" is a party name, and it is plural. The verb is agreeing with the number of the noun, just as is usual in English.