First, I'm not sure how the system works exactly in the UK, but here in the Netherlands, the family of the diseased has the final say about whether or nor his organs are used, regardless of his status as a registered donor.

In that case giving a discount on health insurance won't work, because the family can still wiggle out and then it'll only cost money without increasing the amount of donors. Giving a discount on the funeral would probably work, and I think this is a good plan.

If the family of the diseased has no say in the matter, then giving a discount on the funeral probably wont work, as the dead guy won't care, he's dead anyway. (I know, blunt statement )
Giving a discount in the health care insurance would probably work, but should, in my opinion, only be given if the person is actually suitable as a donor. "Sponsoring" a person that won't be suitable as a donor because of life-style issues (like drug addicts) will only cost money. Of course this checking will cause a huge overhead, so this might not be a good way of stimulating donorship.

Blood donations should not be linked to any monetary advantage for the donor whatsoever, since this will probably lead to people that are not allowed to be a donor (like drug addicts) to lie about it, leading to serious risk for the person receiving the blood.