It is not enough to carry one of the old style donor cards. Like so many things this scheme has effectively been superseded in order for your wishes to be respected you still need to sign up to the modern electronic register. Why? To quote from the official NHS transplant site:
Essentially, the issue is that the card may or may not include your current wishes in relation to which organs can be used. Additionally the card can be lost, stolen or just plain illegible.
Remember that time is important when we are talking about donating organs – this means that in order to remove any ambiguity it is in everyone’s interests that there is a clear instruction on your behalf that specifies which organs can (or cannot) be used. You can also update or amend these wishes at any time. The best way of achieving this is to take 2 minutes to sign up online - please do not put it off, do it now whilst this is fresh in your mind!