In UK they ask about injecting drugs and sleeping with someone who's injected drugs. They also ask about medication, but I have never been asked about cannabis use. I gave blood all through sixth form and uni if they were concerned about cannabis use I'm sure they would have asked specifically then.
I'd say yes you can, but if you are worried just ask them at the donation centre. Cannabis is not the worst thing in the world, I'm sure they'll appreciate you asking!