oh yeah that's just what we want, a health service which provides treatment based on moral judgements rather than actual need. 
As much as there might be an argument for opt-out or for relatives consent to be removed, anyone who actually thinks that unless you donate you should not be able to receive is frankly lacking in intelligent thought.
Where exactly does one draw the line at what constitutes deserving? After all, if you don't work then you don't pay tax, so perhaps you shouldn't receive treatment either? after all, you don't give to the nhs so why should you receive treatment? And who's to say that the person at the top of the list is any more morally superior just because they've said they'll donate their organs in the event of their death? They might be a murderer, or a paedophile, or a deadbeat parent. they might have done a million immoral things compared to your one questionable one - one which might not even come to pass given your organs may never be suitable for donation anyway.
After all, saying you will donate your organs doesn't actually mean it will happen, it just means the thought is there. I might state that I will donate £10000000 to charity in the event of my lottery win. Chances are I'll never win the lottery so won't ever be called on to make that donation.
And this is just it. For most people, organ donation is just a thought. "yeah, if I die they can have my organs, no use to me anyway." If people are prepared to actually opt out of giving organs they have almost certainly put far more thought into that process than the ones who just blindly say they'll do it. Who are we to judge whether someone's reasons are valid or not. Just because they're not valid to you doesn't mean they're not valid to them.