Just completing my Tax Disc online using the new DVLA Beta Portal and it wrapped up with a link panel to the NHS Organ Donation site.
I clicked on it in the kind of what-the-hell-why-not fashion that I am wont to and ended up joining the NHS organ donation register in less than 30 rash seconds.
I hadn't realised how easy they had made it. They cross reference your registration details (name, DOB, Address) with what they already have on you to find your NHS number (you dont need to have it to hand yourself. I mean who ever does?) and they then just put a tick in the database box that says "Recycle His Bits". Couldn't be much simpler.