I am fairly unbothered by organ donation as I know that even with the opt out rule, it ultimately comes down to the next of kin. I have told DH and my DPs that anything can go except reproductive organs. Since having children I can't bear the idea of my womb going anywhere else.
It's also fairly unlikely to happen isn't it? Don't most people die of / in old age and their organs are therefore not of a high enough quality to be donated?
I'm not allowed to donate blood. I did it once, left the centre after the required ten minutes, tea and biccy, and then passed out in the middle of the city centre. Twice. And threw up a lot. Ended up in a&e for four hours! Was told rather strictly that my blood wasn't worth the cost of the ambulance and IV and bed space so please just leave it in future! 