A+
There's no point in having it on your person, your helmet, or wherever, in the UK. They will check your blood group before they give you a blood transfusion - you might be wearing somebody else's helmet, or wristband, or that blood donor card in your wallet might not actually be yours. So they take the time to check it, rather than risk it on your say so, which is absolutely fair enough.
On the other hand, if it makes a driver think of you as human and decide not to spill your blood, it might be no bad thing.