I'm sorry you are having to cope with the loss of your mother. Both my parents died in the winter, and so both times we had one bunch of flowers from the family on the coffin, and the choice was a bit limited because of the time of year. We then had a collection at the church door for charity. When my MIL died, it was January, and we again went for a single bunch of flowers (which cost a fortune - do check with the funeral director and tell them how much you are prepared to pay) and had a charity collection.
If anyone thought we were being mean having just one bunch on the coffins, nobody said so, and in fact, I wouldn't have cared if they had. It is entirely up to you and your father what you do, and if your father would prefer the money to go to Marie Curie, that is his right, and nobody should complain about it. In my opinion, it is much more sensible to let people contribute to a good cause, perhaps a favourite charity of the deceased or of the bereaved, or one form whom the family has had support and help, than to spend a lot on flowers, however beautiful, which will wither in no time and not help anyone. Bear in mind that a good florist will produce a spray which will be longer than a normal bouquet, so that it will not look out of proportion on the coffin.
If you still feel you would rather have more flowers, are there any friends or neighbours with gardens who might be happy to give some flowers? One of the most memorable funerals I went to was of a keen gardener (who died in the summer, which helped) whose garden supplied small bunches of flowers in each pew. The mourners were encouraged to take the bunches home as a small memento of our friend.
I hope your arrangements all go well and that your mother's funeral is a comfort to you all.