Huh. Indeed it is. I'm American, and I did not even realize that "graft" meant "hard work" in British English.
It is understood elsewhere in the UK but not used routinely. It might, say, be deployed for additional effect by someone not from northern England and imply very hard work. However, it might be pronounced in an exaggerated, imitation "generic Yorkshire" accent in which case it gets a bit complicated to explain.
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Actually, isn't the US word "grift" rather than "graft"?
"Grit" is a perfectly valid en_GB word meaning the same (enduring hardship.)