I think it is more complicated than it first appears to donate your body to medical research. The problem with saying take the cost out of the value of your estate is that funeral expenses are up front and probate can take ages.
Prepayment funerals can be a good idea. Prepaying is often money-saving because you pay at today's rates. Obviously, savings depend on how long before the service needs to be provided – but we mostly don't know that. Certainly a big advantage of a funeral plan is that sons and daughters don't have to make decisions at a time of grief, as you will already have done it. I'm sure it's possible to arrange for a cardboard coffin and whatever sort of ceremony you consider appropriate. Cheap and cheerful strikes me as a good way and if I arrange matters myself, no one will be able to accuse my daughter of being a cheapskate or disrespectful. My mum and aunt took out plans at the same time and it made things much easier for me when it came to organising their farewells. As for a warbling minister you don't know – or, more importantly, who doesn't know you – what's wrong with family members saying something? I wrote and spoke the eulogies at said mother's and aunt's funerals. It's incredibly hard but everything I said came from the heart.
Having said all this, I haven't taken out a funeral plan but that is mainly because I am hoping to emigrate and so expect to die on the other side of the world. (I'm hoping to live there for a while first.) However, I am tempted to make a spoof eulogy recording as my one last joke on life.