Nobody has 'ownership' of a dead body. (For example, when some Sheffield Uni students nicked a body during rag week, they could only be charged with stealing the shroud which covered the body, since you can't deprive anyone of what it's impossible to own). So laws determining the type of funeral (or the complete non-existence of one) don't really exist.
As the former MD of a will-writing company I'm very much aware that the provisions of a will, relating to funeral arrangements, have no legal standing whatsoever. (A testator can state that he wants to be interred and even pre-pay for the arrangements himself. If whoever arranges the funeral decides that they'd rather have a cremation, that's what will happen).
If a testator can't make sure of his funeral arrangements (or lack of them) in his will, it seems unlikely that any other document can have any real legal standing. So you could probably over-ride the provisions of the document donating your father's body to science. (However, you might have to pay for the funeral out of your own pocket, rather than from the estate, because your father has effectively made it clear that no part of his estate should be used for funeral expenses).
Just because someone doesn't want a funeral, it's not an indication that they're 'not in their right mind'. My own will requires that I shall be cremated without ceremony and that my ashes shall be disposed of in any convenient refuse receptacle. I regard funerals as totally pointless (and I refuse to attend them).. Your father seems to share my views. Perhaps you should respect his views and (if you so choose) hold a commemorative service to celebrate his life, instead of a funeral?
Chris
PS: Has anyone checked whether Southampton Uni actually want his body? With advances in computer technology, medical students no longer routinely train using cadavers. It's very hard to 'leave your body to