Zmandaz, unfortunately the appointment of a guardian is not set in stone, however if there was a dispute it would certainly bear a lot of weight in court.
The father would have to apply for a residence order under the Children Act. Whoever the child is living with, eg grandparents would be parties to the proceedings.
The court would appoint a CAFCASS officer (sort of social worker) who would do an in depth investigation of the father and grandparent's situation, visiting them at home, observing the child interacting with them, talking to the child alone, the school, any other relevant adults, then write a report into their findings, making a recommendation as to what is in the child's best interest.
In circumstances where the child had an ongoing relationship with grandparents (using this as an example, could equally be close friends, aunt & uncle etc), and little or no relationship with the father, and particularly if moving to the father would mean change of school, friends, missing close relatives etc, then imo it would be more than likely that the child would remain with family, but see the father on a reasonably regular basis so wasn't truly in position of orphan.
As well as the Cafcass report, the court would, as I've mentioned take account of the mother's wishes set out in the guardianship appointment.
The other advantage of doing a will is that rather than all the money being tied up in trust till 18 or 21, the Guardians can access the money to pay for the child's upkeep.