It's worth remembering that nothing written in your will (or otherwise given as an instruction to your friends or relatives) is legally binding in relation to your funeral arrangements. You could state (for example) that you wanted to be cremated without any religious ceremony, but the person arranging the funeral could have your body interred with a Roman Catholic (or any other) ceremony if that was his wish. (Anyone, even a complete stranger, is free to arrange a funeral as long as he ensures that it's paid for, either by him or from the deceased person's estate).
Paying for the arrangements in advance can help to make it more likely that your wishes are carried out but the person arranging the funeral could still substitute their own arrangements if they were that bloody minded!
For what it's worth, my own will states:
"I desire that my body be cremated without ceremony and that my ashes be disposed of, also without ceremony, in any convenient refuse receptacle".
Chris