Well, I'm in the States, so I can only talk about my experiences here. But my MIL's funeral was prepaid and this is how it worked financially - W the help of DH, she planned out the details (so yes, it did involve "funeral plans," as well) and paid the cost at the time. The money went into a fund that made interest, and the interest covered any rise in costs that occurred between the time this was done and the time she passed away. When she passed away, 7 years later, not a penny more was required from her AC and families. So I don't see how anyone can say "poor value." And, ninanthena says, no one had to figure out the details just when they were in the throes of grief.
In our case, though, we made the arrangements through the specific funeral parlor that our family tends to use. We also chose the cemetery, at the time (those costs were included, as I recall). Whether either of those could be changed or not, at any point, I don't know. Nor do I know if there would be any addition or subtraction of costs if we had done so.
Anyhow, DH and I plan to prepay/preplan our own funerals, to make things easier on our DDs and so forth.