Maryz, I don't know whether it applies across the water, but this side there is a sneaky extra thing you need to know.
"Can" actually means "can safely, to a sufficient standard, with reasonable repeatability, without causing severe discomfort".
Reasonable, sufficient and severe are ambiguous of course. But it's a helluva difference from "can at all, once, with extreme pain, and you'll be in bed for days afterwards" literal meaning of "can".
You tick "can't" and then fill in the "additional info" box stating the degree to which you can and the impact or dangers, with examples. So you haven't lied on the form, but you haven't allowed them to glance at the "can" tick and move on without scoring you.
This used to be the DWP's best kept secret. There's slightly better guidance on the 2013 form, but you still have to be on the ball to pick up on it.